Store all redshift PDFs using the same DataHandle

[jtmyles]

This package currently uses a mix of Hdf5Handle and QPHandle objects to store p(z|c), p(z|chat), and n(z), complicating the use of these different PDFs for subsequent analysis like plotting. Storing p(z|c) and p(z|chat) as Hdf5Handles has the disadvantage of fixing histogram binning.

Proposal: transition to using QPHandle everywhere, or alternatively edit SOMPZPzc and SOMPZPzchat to output both the current Hdf5Handles and additionally QPHandles.