Add year argument to public BaseDatasource methods

This is a little thing, but caused a moment of friction when I
was getting started.

The invoke tasks appear to pass a year argument to these methods
and the states that are implemented all have the year argument
in their signature.

If a developer doesn't implement one of the required methods in
the state.datasource.Datasource class, a TypeError is raised
because of the mismatched signature instead of the NotImplemented
exception which would have reminded the contributor which methods
they need to implement in their subclass.
Merge pull request #42 from ghing/dev

Add year argument to public BaseDatasource methods