Add option to save both morph and difference curve

[Sparks29032]

Problem

Currently, post #237, we only have either the option to save the morph or difference curve, not both.

Proposed solution

This requires a conversation about what the file names should be, and how the return of the morph function should be handled.

In diffpy.utils, we have the option to return either the header information or the data table, but not both. This was the result of a conversation where we did not want to confuse the user with a changing shape of returns given options.

For morph, I do think it is fast enough to recompute the diff/morph using the apply since the only real roadblock to computation time is refining. I.e. once you do morph_params, diff_table = morph(..., get_diff=False), you can just insert the morph_params into the new call _, morph_table = morph(..., **morph_params, get_diff=True, apply=True) to quickly obtain the morph.

As such, #237 will maintain the either or return until after we have this discussion.

[Sparks29032]

Big change, affects a lot of code currently using morphpy. Best not to attach this to the 0.3.0 version.