Fix Debug mode

Closes #35

See merge request !160

If I understand it correctly (no documentation in sight), this mechanism
was able to detect whether pointers into the UG3 memory heap were stale,
i.e., did not point to a valid object anymore.  This was used in various
places to verify the data structure integrity. As we have removed the
custom memory heap a long time ago, the HEAPCHECK mechanism cannot be
used anymore (at least to my knowledge you cannot check for pointer
staleness in the system heap).  Nobody noticed this, because the
corresponding assertions were never enabled.  Enabling them leads
to occasional program termination with spurious error messages,
and therefore I herewith remove HEAPFAULT and HEAPCHECK.

HEAPFAULT is some form of consistency check, but I don't really
understand what it is supposed to do.  It may just be a left-over
from the days when UG3 implemented its own memory heap.

The HEAPFAULT in question here triggers an assertion failure
in test-parallel-ug.

It asserts begin < end for a pair of iterators, but the code
right below it handles the cases begin==end, so the assertion
must be wrong.  Nobody notices because apparently nobody ever
enabled the assertion.  Once it is enabled it fires in the
dune-grid standard tests.

The previous way to enable it didn't actually work.

There is an assertion in GetDomainPart that fails when it is actually
enabled.  While this makes it likely that there is a bug somewhere,
the 'part' information is never actually used Dune UGGrid.  I therefore
simply disable the assertion, expecting that this code will be removed
eventually anyway.

Add missing release date for the 2.7.0 release

See merge request !175

... and remove the obsolete comment "unreleased".

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