Remove unused `DataContext.composite` field
Fix AbstractDataContext __or__ and __and__ hooks

The composite syntax for DataContext nodes were updated in c2329675 to
rely on the `composite` key being present with a primary key. This fix
reflects this new requirement.
Add DataContext and DataView clean method

This checks to ensure only a single default template can be defined at
any one time.
Add support for passing the `json` dict directly into __init__

This is shortcut convenience when working with the API directly.