No n_batches in statepoint

I want to remove `n_batches` from the statepoint files because I don't think it provides particularly useful information, and the tally triggers (#298) will make it possible to have inconsistent `n_batches` from different statepoints.  Plus, this will fix the zero-padded problem with #298.

On a related note, do we need to add revision 12 to the statepoint section of the developer's guide?  I don't see what changed between 11 and 12; maybe something in one of the calls to a tally function?