remove output and replace it with assertions.

Note remove_reference is necessary, because you cannot extract the iterator type from
row_reference directly in case: row_reference == row_type&

IteratorFacades::Pointer is assumed to be T* which is not the case for row_reference != row_type&.

Add additional template parameter R (default T&) to handle the case row_reference != row_type&.

This fixes FS#1398 for dune-common.

Use conditional instead.

These headers were moved in Dune 2.3.

The last patch missed a change in the comparison operators. Allocators
compare as equal if memory allocated with one can be deallocate with
the other. With this allocators compare equal only if they are share
the same address.

Cherry-pick from branch eature/FS1429-shared-lib-fixes where it ended up
by accident.

Due to its desgin of using one static memory pool. The allocator
could not be used in different threads at the same time without
side effects. With this revision each instance will hold its own private
instance of the pool.

There should be no issues with this patch and poolallocator seems to be
mostly used in AMG anyway.

when we try to send data, it might happen, that for some reason we don't want to any data.
This happens, when the dynamic size of our CommPolicy is 0. In that case we must not add a
MessageInformation, otherwise we get an MPI error.

Somehow, the vector version of CollectiveCommunication< MPI_Comm >::prod
passed std::plus to allreduce (instead of std::multiplies). This patch
corrects this flaw.

Make type of std::site_t an additional template parameter to avoid the
warning. While this is conceptually redunant its OK since we only use
it in a helper struct.

VariableSizeCommunicator now appears in doxyen and the parallel
index sets are a subsection of the parallel section.

There entries in doxyerror.log about undocumented function
parameters and parameters that had a wrong name in the documentation.
This patch fixes this.

This patch introduces a new macro DUNE_UNUSED_PARAMETER(parm) for marking
unused parameters that are there for a purpose (e.g. the prescribed interfac
The macro simply performs a cast to void. In addition all unused parameter w
that I found were removed.

Thanks, Oliver!

The new YaspGrid constructors require an array with the number of cells
in each direction. Unfortunately, there is no array constructor that
takes a single element and copies it to all array elements.

This patch adds a simple function Dune::fill_array() to do just that.

With this patch VariableSizeCommunicator now really supports a varying number of data entries
per index during communication.