Implement ITMQC-XF methods in JCP, 156, 174109 (2022)

[jkha-unist]

No description provided.

[jkha-unist]

Actually, it's not based on that commit, sorry

[jkha-unist]

I'm gonna re-open another pull request and write commits in the bottom-up passion to emphasize the code changes and prevent any confusion from possible duplications of commits.

[jkha-unist]

Issues

1. change to state-pairwise expression or give option.
2. calculate decoherence term in shxf.py and give it to el_propagator, i.e. every nuclear time step, not electronic time step? (need change in all xf files).