Add Var.replace

[ph-kev]

closes #123; This PR adds Var.replace which can be used to replace all missing and NaN values in data with another value. This use case for this function is dealing with land/ocean simulations where values in the ocean/land are missing and NaN values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.20%. Comparing base (a189286) to head (259ccc4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #132   +/-   ##
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  Coverage   98.19%   98.20%           
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  Files          11       11           
  Lines        1110     1116    +6     
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+ Hits         1090     1096    +6     
  Misses         20       20           

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