Some small changes to facilitate smooth PyPi integration:
* update the Python installation guide
* update the license file
* add a `setup.cfg`
* add a stub for the Python readme

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 .

Improve the Python wrapper generation and installation:
  - install a proper module that can be extended with Python code;
  - give the user more control over where to install the module (e.g. as a user package or in a virtualenv).

During building, the following sub-directory is built in the build director (`CMAKE_BINARY_DIR`)
```
└── python
    └── arbor
            ├── __init__.py
            ├── arbor.so
            └── VERSION
```
This path can then be copied VERBATIM to the target installation path. By default this will be in `CMAKE_INSTALL_PREFIX/lib/python%d.%d/site-packages`.
An additional CMake parameter `ARB_PYTHON_PREFIX` can be used to specify an alternative destination for installing the Python module.

The Python part of the wrapper, implemented in `__init__.py` is currently very limited, only providing `__version__` and `__config__` variables.

The installation guide was updated to cover the Python installation.

A big update that wraps over 90% of the public C++ API, with enough functionality to let Python users to perform useful modelling.

Key features
- wrapping of cable cell functionality
  - user-defined explicit compartmentalisation not supported
- `single_cell_model` abstraction unique to the Python wrapper that simplifies developing and testing single cell models
- one-dimensional cell builder for Python wrapper that simplfies building cells that 
  - in and of itself limited in scope, but a very useful example of mapping a richer morphology builder onto `sample_tree`s.
- parsing of region and location expressions from strings
  - implements a generic s-expression parser that we can use for other purposes later