Python Documentation PR (#687)

Update documentation for Python.

    splits the conceptual model ideas from the C++ docs into their own section
    has C++ and Python docs for recipes, domain decomposition, etc.

fixes #667

Added the following documentation (structure):

GETTING STARTED:

    Installing Arbor/Requirements/Optional Requirements/Python
    Installing Arbor/Building and Installing Arbor/Python Front End

MODEL BASICS:

    Overview
    Common Types
    Recipes
    Domain Decomposition
    Simulations

PYTHON:

    Overview
    Common Types
    Recipes
    Domain Decomposition
    Simulations

DEVELOPERS:

    Python Profiler
    Python Unit Testing

GETTING STARTED has two added sections of optional requirements using python and how to build the python front end.

MODEL BASICS describes Arbor's concepts in general (independent of programming language), thus general information on concepts in C++ API was moved here/ added.

PYTHON describes Arbor's python frontend in the same structure as MODEL BASICS and C++ API ( needs updates as soon as features are added/changed in new python PR).

DEVELOPERS section has two added sections for meter management and unit testing with python front end.

Further, some corrections in existing documentation (for obvious errors, e.g. duplicate text, not ending sentences) and referencing sections were done.