Implementation of Community object

[davidorme]

This issue is to create a PR to provide an implementation of the Community class. It is intended to isolate that particular component of @AmyOctoCat's work in progress PR #231 to allow a review and refactor of that section of the code and allow us to bring in that code in smaller chunks.

This issue should:

• provide something close to a final definition of the Community building on @AmyOctoCat's work and review by @davidorme,
• methods to create instances from data in files - and also handle multiple communities in files.
• test that code.
• provide API docs

Wider user docs can be delayed until more of the system is in place.