Cross terms

[pjgrandinetti]

Changed cross-term coefficients for quad-csa and quad-coupling. Still need to confirm the details, but the stmas spectra appear to give better agreement with expectations. Wrote an example notebook for a C13 MAS spectrum with C13-N14 coupling, but did not get expected 4th-rank spectrum. Also, couldn't work out how to get the C13-N14 example to appear in the Sphinx Gallery.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1d39bdf) 100.00% compared to head (71e0147) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #307   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          115       115           
  Lines         7652      7652           
=========================================
  Hits          7652      7652           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[deepanshs]

There was a bug that disabled all quad-related frequency computation when the method had a channel with I=0.5. That was the reason why the example didn't show any 4th rank lineshape. After the fix, you can see the 4th rank line-shape in the example.

[pjgrandinetti]

It looks like the dipolar-quad cross-term frequency contribution is included for the m_I = +/- 1 states of N14, but it doesn't seem to be working for C13 coupled to the m_I = 0 state of N14. This transition should be a scaled mirror image of the other transitions. I didn't see where this could be missing in the code.

[deepanshs]

I am merging this PR. There is still some work that needs to be done here, but that can go in as a separate PR.