Introduction of aggregation datasets

[davidhassell]

Fixes #319

Moves CFA functionality from cf-python to cfdm

I think fairly self explanatory (let me know if otherwise :)), but some points of note:

• CFA API is updated a bit to reflect that which is (currently) going through discussion over at the CF conventions
• read.py is refactored as a class so that upstream applications (e.g. cf-python) can avoid large amounts of docstring and code duplication
• The inheritance structure of Array objects has been changed for similar reasons. This includes moving the old FileArrayMixin to a the abstract FileArray