Make compile again and some convenience

Again, not tested, cross your fingers...

This patch makes it possible to use the 2.3 release of MultiDomainGrid
with DUNE 2.4.

The fix in the original patch introduces a messy bug in Dune 2.3, as
with this patch, the geometry returns a temporary object, but the facade
class  still returns a const reference. That reference now points to the
temporary, which goes out of scope as soon as the facade function
exists. As a result, the user gets a reference to some random stack
memory. Unfortunately, this will normally not trigger a crash; instead,
calculations using the jacobian will simply yield wrong results.

Kudos to Thomas Fetzer for finding the problem.

This reverts commit 62c359d0.

Clang warns about it.

Thanks to Christoph Grüninger for reporting the problem!

Remove commented lines for autotools, too.

[MultiDomainMCMGMapper] Fix broken implementation of contains() with a codimension template parameter

The mapper in dune-grid has recently been converted to store the
per-GeometryType offsets in a std::vector indexed by the
GlobalGeometryTypeIndex, which improves lookup times by replacing an
expensive map lookup with a simple array access.

This patch does the same thing for the MultiDomainMCMGMapper and also
gets rid of the separate size variable - that information has been moved
to the end of the offsets array, where it is automatically built as part
of a call to std::partial_sum, converting per-GeometryType sizes to
offsets.

Thanks to Christoph Grüninger for bringing this up!

[Documentation] Fix link to core modules and only list real dependencies, not all of the core modules

This reverts commit 9e34b076, as those
specializations are still needed for the 2.3 release series of DUNE.

GitHub wants a protocol specifier to recognize global URLs.

- Improve readability of first two levels of headings
- Add .md suffix to documentation files to trigger markdown formatting
  on GitHub.

This patch finally adds something like a minimum amount of
documentation, providing

* links to a canonical source (GitHub)
* proper license information (GPLv2 with template exception and LPGLv3
  or later)
* a list of contributors
* release notes
* a sane versioning scheme (reuse first two components of compatible
  DUNE release)

The documentation has been formatted in GitHub-flavored Markdown to
enable nice formatting on the GitHub site.

Apparently you can't assign an iterator to an EntityPointer during
construction. Well, broken engine concept, so I don't really care...

Missed an iterator dereferencing and method rename during the recent
cleanup...

This is kind of pointless, as they will go away directly after 2.3, but
without them, it's not possible to build the gridcheck.

Some fixes to enable compilation of the gridcheck

- make constructors public
- add default constructor