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

Otherwise CMake will fail anyway.

This allows to use the build system without using dunecontrol and
additional settings for other modules. We simply assume that dune-common
is located similar to this module.

Disable xdr, unless HAVE_RPC_RPC_H is set

Closes #33

See merge request !82

Mention the new TET_RULESET flag in the changelog

See merge request !80

bump version to 2.7-git [ci skip]

See merge request !79

[ci] stop testing with Debian 8

See merge request !77

Allow full tet ruleset

See merge request !75

'Broadcast' wants a plain pointer, a std::unique_ptr won't do.

UG contains two implementations of green closures of tetrahedral
elements, and the TET_RULESET macro allows to switch between
the two.  As I recently learned from Peter Bastian, we have been
using the inferior implementation all along, and we should be
enabling TET_RULESET in the future.  As a first step, this patch
allows to set TET_RULESET from the CMake flags.

Incidentally, setting TET_RULESET fixes

  core/dune-grid#27

Beware: when TET_RULESET is set, UG wants to read a table from
the file RefRule.data (in uggrid/lib/ugdata).  This file needs
to be in the current directory, otherwise UGGrid will abort.

Remove unused method YAlignment

See merge request !74