Move the PDB, VTK, SLD, and OMF readers to sasdata package

[krzywon]

Is your feature request related to a problem? Please describe.
This might be better as a discussion, but I think the various readers (PDBReader, etc.) and SLD profile classes (MagSLD, etc.) used by the generic scattering calculator would fit better in the sasdata package instead of sascalc. This would also help clean up sas_gen.py and allow outside groups a way to separate the readers from the calculations.

Describe the solution you'd like
Create new readers in sascalc.dataloader.readers for each of the types, move SLD data structure classes to data_info.py or a similar file, and add a separate loader class that returns SLD profiles instead of lists of Data1/2D.