workflow reorganization

[joshspeagle]

This is part of a larger initiative to reorganize the workflow so that most of the computation is internal. This way a user should just be able to do something like:

import object
obj = object(data, params)
object.fit(new_data)
object.compute_pdf(params)
zpdfs = object.zpdfs

This should enable a very clean workflow for general users, while advanced users can still access internal quantities for more advanced analyses.