Add python interface for ELPA
The ELPA C API is wrapped by cython code that can be called from python. Python classes for convenience access are provided. Tests are included and activated in CI.
The interface is built when --enable-python is given to configure. The requirements then are then python, numpy, mpi4py, cython. The tests can be enabled with --enable-python-tests; then, an additional requirement is pytest.
Currently, the interface is only tested with the MPI version of ELPA.