Work around issue caused by some versions of pip skipping the `install` stage when no additional command line args are passed via `--install-option`.

Keep settings that can be modified via install option in a singleton that initializes them to their default.

* add MANIFEST file that describes all paths required in source distribution so that pip can compile Arbor.
* fixes to `setup.py` to help it work across supported Python versions.

Tested with:
* setup tools and pip
* python 3.7 and 3.8
* pip 19 and pip 20
* Linux and OS X

Add a `setup.py` for installing Arbor directly with pip/setuptools.

Implement a setuptools extension for CMake in `setup.py`. To be honest, I don't understand entirely how it works, but setuptools is obtuse enough that I don't feel at all guilty about this.

Define additional flags for optionally enabling GPUs, MPI, Vectorization and micro-architecture targets, for more adventurous users.

The documentation is updated with a "howto pip" for more casual users who don't want anything to do with CMake.

Fixes #958 .