[WIP] Build system changes to allow pip installation from source

[tilmantroester]

This PR attempts to unify the build process such that a single python setup.py install builds and installs the whole library. Ditto for pip. To accomplish this, libchord.so is now being compiled at the build_py step (for stupid, setuptools-related reasons) and then copied into the package. The extension is compiled with the right rpath, $ORIGIN, and install_name settings so that things work. Getting this to work cost me a good chunk of my sanity and will probably leave me with setuptools related PTSD for the foreseeable future.

To summarise:

• python setup.py install, pip install ., pip install git+https://github.com/tilmantroester/PolyChordLite.git@compile_stuff_for_pip#egg=pypolychord seem to work on both Linux and macOS for MPI.
• A no-MPI install is also possible by setting the --no-mpi flag. Note that the flag position is after setup.py, e.g., python setup.py --no-mpi install, for stupid, setuptools-related reasons.
• There's now a python setup.py clean command that runs make veryclean

This should probably be tested in more detail. At the moment it is based of the setup_pip branch but I think it could easily be rebased to master.

[williamjameshandley]

Potentially fixes #13 and #11

[tilmantroester]

Should be replaced by PR #24.