[bugfix][Python] Revert cleanup in locking.py.

See merge request !1063

for portalocker is still needed.

Cleanup/python

See merge request !1050

[cmake][cleanup] Reduce output for cmake tests from module deps

See merge request !1061

[python][cmake] Do not write requirements.txt file

See merge request !1062

[cmake][mpi] Use correct guard MPI

See merge request !804

The dune CMake configuration actually need `MPI_C_FOUND` to be true. If this is true `HAVE_MPI` is set. This is used with this commit.

According to the CMake 3.1 doc `MPI_FOUND` is actually discouraged. It is later reintroduced but is only true if MPI has been found for all requested languages. Which is also not what we want. 

[cmake][cleanup] Remove search status message erroneously printed for all dependencies (also those not found)

[Python] avoid logging both 'generating' and 'compiling' module in two lines

See merge request !1057

Move dunecommon target to the root CMakeLists file

See merge request !1048

[python] Add preprocessor option to disable Python deprecation warning

See merge request !1043

Feature/python reduce printing compiling

See merge request !1052

Reorganize imports to avoid circular dependency

[python][cleanup][lock] Remove fallback Lock implementation since portalocker is a required dependency

[python] Add mechanism to register external Pyhton modules to dune-py

Closes #274

See merge request !1049

The function dune.common.registerExternalModule(module) has
to be actively called in the downstream module to register
an external package. This triggers the reconfiguration of dune-py
in case dune-py has already been created.

Add missing includes CMakePushCheckState and FeatureSummary

See merge request !1047

Redesign the cmake dune_add_library function

See merge request !1037

Use keyword extends in gitlab-ci

See merge request !1046

Deprecate cmake function remove_processed_modules

See merge request !1039

[math] Use integer type in Factorial

See merge request !1045