CNG: Point cost functions are now based off of inverse square weighting

[rcjackson]

The point observation cost function and gradient, instead of using a constant weight for each grid point over a radius of influence from the point observation, now use weights based off of the inverse square of the distance between the grid point and the observation. This is a common technique used for spatial interpolation of point observations for GIS and weather maps and better takes into account the decreasing influence of an observation on the data assimilation the further away it is from the point.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.06%. Comparing base (9741941) to head (3f30db0).
⚠️ Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   70.00%   70.06%   +0.06%     
==========================================
  Files          32       32              
  Lines        5340     5351      +11     
==========================================
+ Hits         3738     3749      +11     
  Misses       1602     1602              

Flag	Coverage Δ
unittests	70.06% <100.00%> (+0.06%)	⬆️

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