Begin CMake front-end for Python packaging machinery.
Allow gmxapi Python package sources to be discovered in the main GROMACS
CMake configuration step. During build, a functioning package will
appear in the build tree in
${CMAKE_BINARY_DIR}/python_packaging/src/gmxapi_staging that can be used
for tests and auto-generated documentation builds.
A Python 'sdist' can be built from this location in the build tree, or
the package can be built and installed directly from the source tree
with setup.py in python_packaging/src/
Refs: #2896
Change-Id: I225dd890c93405880ef057465eb7c159cd097665