Compute geometric and geopotential height

[sandorkertesz]

This PR adds the following methods to the vertical module:

• geopotential_height_from_geopotential(z)
• geopotential_height_from_geometric_height(h, R_earth=constants.R_earth)
• geopotential_from_geometric_height(h, R_earth=constants.R_earth)
• geometric_height_from_geopotential_height(gh, R_earth=constants.R_earth)
• geometric_height_from_geopotential(z, R_earth=constants.R_earth)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.76%. Comparing base (f8defab) to head (6d28866).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #39      +/-   ##
===========================================
+ Coverage    98.68%   98.76%   +0.07%     
===========================================
  Files            8        9       +1     
  Lines          684      727      +43     
  Branches        26       31       +5     
===========================================
+ Hits           675      718      +43     
  Misses           7        7              
  Partials         2        2              

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