dunecontrol: .git does not have to be a directory

See merge request !289

`.git` can be a regular file when using git-worktree.  This change
makes `dunecontrol status` work for me again.

.gitlab-ci.yml: update environments

See merge request !288

 - Added:   Debian 10 with gcc in C++17 mode
 - Removed: Debian 8 + Backports with clang

Reference: http://lists.dune-project.org/pipermail/dune-devel/2017-July/002224.html

include <cstddef>

See merge request !286

Rationale: The macro `offsetof` is officially defined in <cstddef>.

resolve ambiguous call by explicitly calling `Dune::Hybrid::size`

Closes #77

See merge request !283

Closes: #77

[bugfix] multiply by field_type instead of value_type

See merge request !274

Update dunecontrol doc

See merge request !271

The proper vector space operation is multiplication by a scalar, not by a vector
component.

It has been removed several years ago.

Clean up `#include`s and `#if HAVE_MPI` in mostly `dune/common/parallel/`

See merge request !244

Feature/add exadune tbbtest

See merge request !253

Added missing FieldTraits specialization for DiagonalMatrix

See merge request !269

[bugfix] adapt web link in dunecontrol man page to new homepage

See merge request !270

Add MPI_NO_CPPBIND to compile flags to deactivate C++ with MPT

See merge request !267

[fix] deduce the VC return type instead of hardwireing them

See merge request !264

[cmake][python] Check that the get-pip.py script was successfully fetched

See merge request !254

doxygen: enable SEARCHENGINE

See merge request !262

The JS-based search engine is enabled for the doxygen documentation on
the web site.  This change also enables it by default for local builds
of documentation.

CollectiveCommunication: use `const` references or pointers where possible

See merge request !261

CMakeLists.txt: run MPI tests in parallel

See merge request !260

In older MPI versions the send or input buffer was a `void*`.  This
was changed to a `const void*` in newer versions.

DUNE's `CollectiveCommunication` should not expose the missing
`const`, but instead just cast the `const` away internally to be
compatible with the old MPI interface.

This change makes the test suite run the MPI tests in parallel.

The change also runs the `mpicollectivecommunication` test when
building without MPI as the `CollectiveCommunication` wrapper should
also work in a sequential build.

Remove (index|integer)_sequence fallback to fix #73 in master

Closes #73

See merge request !256

[Fix] Update mailing lists addresses.

See merge request !258

make `TestSuite::operator bool()` explicit

See merge request !255

Implicit conversion to `bool` allows using `TestSuite` in arithmetic
expressions such as `1.0 / t` where `t` is an instance of `TestSuite`.
With an explicit conversion operator, one gets a compiler error when
using the wrong variable here.

All supported compiler/standard library combinations have it (so that
this change does no harm)
Libc++, however, does not declare __cpp_lib_integer_sequence, causing
the fail to check and the fallback to be used. This leads to problems
because code is already freely mixing Dune::Std::*_sequence and
std::* expecting them to be the same, which with Libc++ they are not.

Fix type of returned references and thus #68.

Closes #68

See merge request !245