fix delta k in RSD velocity gradients for non-cubic boxes

[daviesje]

@binnietom found a bug in the brightness temperature RSDs which uses the wrong delta k for non-cubic boxes.

Fixes #403

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.47%. Comparing base (1df46a8) to head (a16376c).
Report is 29 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #410   +/-   ##
=======================================
  Coverage   82.47%   82.47%           
=======================================
  Files          13       13           
  Lines        3121     3121           
  Branches      842      842           
=======================================
  Hits         2574     2574           
  Misses        391      391           
  Partials      156      156           

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