More strain less stress

[smribet]

This PR is to update the strain branch by cleaning up the structure and adding more processing and plotting options.

[smribet]

The linter is failing on the functions that are in py4DSTEM/process/latticevectors.
@bsavitzky do you want to keep these functions or can I remove them? If we keep them, I just need to do a bit of restructuring.

[bsavitzky]

This module is now functional - i.e. the bugs which prevented strain mapping from working at all with calibrated braggvectors have been fixed - however, significant testing is still required, and in particular QR rotation handling needs to be reviewed and thoroughly tested. All that said, I'm merging this so that users who are currently stuck can proceed. Anyone using this module should plan to confirm the correctness of QR rotations manually for your datasets until the next update to this module is released. Thanks to everyone for your patience....