dune-python migration

As decided on the Dune developer meeting, I will migrate the python build system infrastructure currently located in dune-python to dune-common. I will try to split the migration into 3 parts:

• !218 (merged) is kinda unrelated, but used in the project (for sake of simplicity, this MR builds upon that branch)
• This MR will contain all the stuff that I consider useful for all python related projects, such as
  • finding interpreter and libraries
  • checking availability of python packages
  • defining installation rules for python packages
  • run python test commands during ctest
• An upcoming merge request (!220 (merged)) will implement everything that relates to the configure-time envsetup currently used by projects that need to run python from cmake.

To grasp the changes of this MR:

• DunePythonCommonMacros is the entry-level module, where all the logic that is executed during configuration of dune-common is located
• All the cmake modules prefixed with DunePython contain code that is intended to be used by users. It is documented with Sphinx.