in order to make GMPField work in more situations, we add a constrctor, which matches
all scalar data types, which can me converted to K.
We deliberately exclude K from the template match.

we first import function like std::max into the local scope and the call max(a,b) instead of
std::max(a,b). This allows to match specializations for other data types via ADL.

The templated constructor of GMPField fatched everything. We reduce the constructor
to those scalar values, which are convertible to mpf_class. As this removes the constructor
from char* we explicitly add constructor from char* and std::string.

This is the main part to fix FS#1665

most of the tests work. std::complex<GMPField> does not work as expected, but this is
perhaps something we don't want to support. As std::complex does not allow to initialize
with other parameters than the data type itself, we can not initialize from int or other
compatible scalar values. The smae problem occures when interacting with int values directly.

Fixes FS#FS#1649.

This closes FS#1651.

ptrdiff_t is a signed integer type and so the expression
  (std::uintptr_t)(ptr) % page_size
could become a negative value. In this case the page_ptr would be the
address of the next page after the allocation.

This wrong behaviour could be observed on (32bit) PowerPC: here
ptr was 0xf78cfe00 and page_ptr was calculated as 0xf78d0000 instead
of the correct 0xf78c0000.

The std::numeric_limits specialization for bigunsignedint requires
access to the internal state of bigunsignedint. Previously, the correct
specialization of std::numeric_limits was a friend of bigunsignedint,
but that creates problems on recent versions of clang with the
alternative libc++ library, because that library declares the base
template of std::numeric_limits as a class and clang subsequently
complains if the friend declaration uses 'struct'. Unfortunately,
libstdc++ uses a struct, making it impossible to keep clang happy for
both standard libraries. So we introduce a helper class that provides
access to the internal state and which now becomes a friend of
bigunsignedint. The numeric_limits specialization inherits from the
helper to use it.

According to the standard, all specializations of std::hash must export
argument_type and result_type [20.8.12/1]. The version of libc++ (LLVM's
alternative C++ standard library) tripped over this requirement on my
machine, so this patch adds the typedefs.

With just the two overloaded constructors

  bigunsignedint(int)
  bigunsigendint(uintmax_t)

trying to call the constructor with any integer type T different from
int and uintmax_t results in an ambiguous call: the promotion of T to
either int or uintmax_t are ranked the same. (Note that this does not
depend on the specific choice of int and uintmax_t.)

This patch provides a templated constructor for all signed integer
types which is ranked over the constructor taking an uintmax_t, making
overload resolution unambiguous for signed integer types. By also
"hiding" it for unsigned types using enable_if, the overload
resolution for unsigned integer types is also unambiguous; there is
only a single viable candidate left: the one taking an uintmax_t.

Use the fixed-width integer types introduced in C++11 instead of
making assumptions on the size of integers. The following changes were
implemented:

unsigned short used to store the bigunsignedint:
  Changed type to uint16_t. The implementation requires this to be a
  16-bit type.

int, unsigned int used to store intermediates:
  Changed type to (u)int_fast32_t. This is enough to hold sums, products
  or differences of two uint16_t integers.

std::size_t used as parameter in constructor and arithmetic operators:
  Changed type to uintmax_t. Any unsigned integer type should be usable
  in conjunction with bigunsignedint.

Return value of touint() member function:
  Changed type to uint_least32_t. The current implementation only
  returns the last two "digits" (of 16 bits each).

This change should make formatString safe to use in multi-threaded
programs.

This fixes FS#1636.

Before this commit communcation would be initiated even on empty
interfaces. While this does not lead to overwriting data or producing
segmentation faults, it does lead to accessing the first component of
empty vectors to get a pointer to pass to MPI functions. This produces
error output with valgrind and other memory debuggers. Therefore with this
commit we will first check whether the interface contains values and do
nothing if it doesn't. We also augmented the test to catch the case wher only
some processes have an empty interface.

This check was added to improve the error message returned when using
MPI without correctly initializing it (e.g. by creating a parallel
grid), see FS#1612 for further information.

[Typetraits][Compatibility] Provide partial backwards compatibility for is_indexable on old GCC versions

Older GCC versions have some nasty bugs related to SFINAE and indexing
with operator[]. This patch adds a configuration check for those
problems and a partial backwards compatibility hack for those older
compilers that is sufficient for our current use case in the VTK
writer.

GCC 4.4, I'm looking at you

is_indexable can be used to test whether a type can be indexed (via
operator[]).

@Markus: too much CMake?! ;-)