MakePythonAPI: Produce Python interfaces to the C++ libraries

* The Python interface to a shared library is made via MakePythonAPI module.
  - Moved installation-related parts from MakeSwigLib to MakePythonAPI;
  - Moved Wheel-related parts from MakeSwigLib to MakePythonAPI.

* Differentiate between `BORNAGAIN_PYTHON` variable and `WITH_PYTHON_API`
argument:
  + BORNAGAIN_PYTHON = Add Python-related functionality to the C++ libraries;
  + WITH_PYTHON_API = Add a Python interface to the C++ libraries;
    currently made with SWIG.

* Add OUTPUT_DIR to MakeSharedLib to allow setting an output folder for the shared library.