Simplify Datasource

[mdr223]

This PR covers changes which simplify the use of the DataSource class for:

1. Users writing programs (especially with validation data), and
2. The internal management of DataSources in PZ

The primary changes I made were to:

1. Reduce the three data source classes (DataSource, AbstractDataSource, and ValidationDataSource) down to a single DataSource class
2. Limit the number of methods the user needs to implement to just __len__() and get_item()

This in turn led to a number of other changes / simplifications in the code including:

1. Using datasource directly in the scan operators (both logical and physical) instead of dataset_id
2. Changing source_id --> source_idx everywhere (this eliminated some weird renaming logic)
3. And various other small fixes which resulted from these changes

Finally, I added a large set of documentation for the DataSource class(es)