Nimisha/split converge api

We will need to go through and update all the docstrings in Mixed, Draft, Split, Split_Draft, ModuleStoreRead, ModuleStoreWrite.  Perhaps we should keep the API docs only in one class, Mixed, and have all the others refer to Mixed.  They only need to override the docstring if they have anything additional to add.  I suggest Mixed since it's the API that all higher layers will use.