Fix test for small rho

[jcapriot]

Checking for small k**2 was too aggressive, and would catch things when rho was big relative to a...

Instead check rho2 / (radius2 + z**2). I.e. check if the angle Rho-Z-Radius is small

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.93%. Comparing base (3b1f8d5) to head (4cb4a17).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #81   +/-   ##
=======================================
  Coverage   85.93%   85.93%           
=======================================
  Files          31       31           
  Lines        3149     3149           
=======================================
  Hits         2706     2706           
  Misses        443      443           

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