Python bindings for reference model

Installable with `pip install .` in the repository's root directory.

The Python package is built as an .so, which setuptools copies into
py_package/ and installs.

Signed-off-by: Kaushik Varadharajan <kaushik.varadharajan@arm.com>
Change-Id: I20030c02bab6ca0357b55422fc7066ac3a6f6546