Remove support for _2 and _3 macro in dune-grid

See merge request !174

Use [[maybe_unused]] instead of DUNE_UNUSED

See merge request !173

Feature/minor fixes

See merge request !172

Use named struct

See merge request !169

This fixes Clang 11's warning: anonymous non-C-compatible
type given name for linkage purposes by typedef declaration;
add a tag name here [-Wnon-c-typedef-for-linkage]

[ci] Allow failure for Debian 11

See merge request !171

Compiler has an ICE

remove ugdevices from dimension-dependent targets

See merge request !168

Stabilize ddd type definitions

See merge request staging/dune-uggrid!167

The DDD_TypeDefine method previously required pointers to the individual
data members of a data structure to compute its layout.  For this you
need a pointer to the beginning of the data structure, and the code
used stuff like nullptr and pointers onto the stack for this.
This worked (mostly), but working with objects starting at 0
involves undefined behavior.

This patch makes the code more robust by using the standard method
'offsetof' to compute the data structure layouts.  This is done
outside of DDD_TypeDefine, which hence changes its interface.
This is not much of a problem, because the method is only used
in initddd.cc anyway.

Cleanup the cmake structure

See merge request !165

replace individual object libraries by common dimension-dependent library duneuggrid_xd and one exported target duneuggrid

cleanup order of sources in target_sources and remove enabled_dimension variable

rename dimension dependent libraries

Add several missing communication interfaces

See merge request !159

Remove periodic grids

See merge request !163

This was never used, and it is of no interest to Dune.
So let's remove it.

Fetch default yml files via different project instead of url

See merge request !161

This is a workaround for a bug that showed up after gitlab migration. See issue
infrastructure/issues0#62 for further discussion.

These are needed to implement the Dune InteriorBorder_All_Interface.

Merge branch 'feature/require-cmake-3.13' into 'master'

See merge request [!158]

  [!158]: Nonestaging/dune-uggrid/merge_requests/158

Merge branch 'remove-evalproc' into 'master'

See merge request [!157]

  [!157]: Nonestaging/dune-uggrid/merge_requests/157

It is not used.

The flags have been removed from the code a while ago.

It was used to distinguish headers that were internal
to UG.  However, today all code in dune-uggrid is
internal to the Dune grid interface, and therefore
the old distinction is not needed anymore.

Merge branch 'remove-directory-numerics' into 'master'

ref:staging/dune-uggrid\> It is not used anymore.

See merge request [!156]

  [!156]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/156

It is not used anymore.

Merge branch 'remove-more-unused-stuff' into 'master'

See merge request [!153]

  [!153]: Nonestaging/dune-uggrid/merge_requests/153

Instead, we let the multigrid store them directly.  That is
much simpler.

Apparently they were not used at all

And remove lots of code that we never actually exercise.

Our format does not use type options.