Update documentation.

[veronicaguo]

• Add model import.
• Change running model command to m.command('start').

[mikofski]

@veronicaguo I think we should add or update the announements on index.rst to say something about how v0.3.1 will not work without parameters, so this will break old models, they will have to migrate to use parameters.

Also add something to talk about class Meta in the announcements but also somewhere else in the docs.

And we should mention the incompatibility with the Pint v0.8.

[mikofski]

I think we should update the API as well with some of the new classes and auto-document them.

• in data-sources.rst add new DataParameter

  .. autoclass:: DataParameter

• ditto for outputs.rst

• and formulas.rst, calculations.rst and simulations.rst

[mikofski]

@veronicaguo Thanks for making those changes. This is looking good. Let me work on it today, maybe I can wrap it up. :)

[mikofski]

fixes #95