Add FindPython module

Add a wrapper for CMake's FindPython module. This replaces the existing FindPythonLibs and FindPythonInterp modules. The module caches the major Python version to speedup the lookup for subsequent calls.

Beyond the version caching, I did not yet implement any further caching as was done e.g. for FindPythonInterp in !206 (merged). I would like us first to move to cmake 3.16.x as there have been quite a few improvements upstream in that area before we implement our own caching.

The new code has been tested with a few athena packages. As a second step I would then migrate all FindPythonInterp/Libs calls to this new module.

AE Build SUCCESS
Build logfiles are available at Jenkins [AE-MERGE-REQUEST-CC7 #142]

merged

mentioned in commit f53e5c31