add tensor convention utilities

[pjgrandinetti]

• [x] RTD docs
• [ ] Update origin offset from larmor_freq to B0_to_ref_freq function.
• [x] Update changelog
• [x] Add test coverage

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.99%. Comparing base (19eda42) to head (02f4d0c). Report is 93 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files         124      124           
  Lines        8586     8599   +13     
=======================================
+ Hits         8500     8513   +13     
  Misses         86       86           

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