these variables were used to chnage the memory mangement of a particular
UG instance. As these variables were global, this could only work by
coincedence. We moved them to the heap, as this is the memory mangement
unit for the multi grid.

  - we compute some sizes, but the name was misspelled, thus we always used the default

Cleanup

See merge request !98

The rank will no longer be readily available as a global variable once
a context object for PPIF is used.  Also MPI implementations already
provide a way to tag output with the rank of processes, for example
OpenMPI's `mpirun` has the `--tag-output` and `--output-filename`
options.

replace `ArrayOf` implementation with `std::vector`

Closes #17

See merge request !71

clean up DDD and PPIF a bit

See merge request !95

It would not have worked correctly anyway as output might still linger
in buffers before the next process starts writing.  Using per-rank
output files is a better solution, for example OpenMPI mpirun's
`--output-filename` option.

This also allows removing the `_context` member variable from
`DDD_CTRL` which cannot be shared between multiple grids (that might
use different MPI communicators in the future).

The alternative using stack-allocated `MPI_Request` had to be removed
as it assumes `MPI_Request` is implemented as a pointer.  If stack
allocation is wanted, `msgid` should be made a type alias for `Msg`
instead.

DUNE will initialize MPI for us.

This also changes the value of `NO_MSGID` to a null pointer as that is
the only value that we know cannot be returned otherwise.

Feature/support mingw

See merge request !57

Fix compiler warning due to const qualifier in return by value

See merge request !91

[cmake] Fix compatibility issue with CMake 3.10 leadig to -D-pthread

Closes #34

See merge request !88

CMake 3.10 overhauled FindMPI.cmake. Now -pthread ends in various
MPI-related variables and from there in MPI_DUNE_COMPILE_FLAGS. With
COMPILE_DEFINITIONS -pthread got the prefix -D.
Changed to COMPILE_OPTIONS which does not add -D to the passed flags.

Fixes #34

Signed-off-by: Christoph Grüninger <christoph.grueninger@iws.uni-stuttgart.de>

Fix building with UG_ENABLE_DEBUGGING=ON

Closes #23

See merge request !86

Missing namespace qualifiers and include directives.

- They don't compile.
- They are not used.

They are only required if 'Debug' is set, that's why nobody
has missed them so far.

Buildsystem fixes

See merge request !84