Move Perspective Calculation Files into sascalc

[krzywon]

Going through the sas/sasgui/perspectives folder I've identified a few python files that should probably be in sascalc rather than sasgui.

calculator: load_thread.py, resolcal_thread.py
fitting: calcthread.py, fit_thread.py, model_thread.py, models.py, pagestate.py (also move SimFitPageState class from simfitpage.py to pagestate.py)
invariant: invariant_state.py, invariant_details.py (this should be called invariant_thread.py)
pr: inversion_state.py, pr_thread.py

There may be other individual methods that need to move. I will update this ticket as they are found.

This is related to #295.