Add Mesh object

[PProfizi]

Fix #234.

A Mesh instance is held by a Simulation instance and wraps a core.MeshedRegion instance.

The goal of the Mesh class is to wrap MeshedRegion with an API which is flatter, more Pythonic, more user-friendly, and adds Post-specific additional mesh manipulation capabilities such as mapping.

[codecov[bot]]

Codecov Report

Merging #226 (fb4ea80) into post_refactor (89fdadf) will increase coverage by 3.44%. The diff coverage is 100.00%.

@@                Coverage Diff                @@
##           post_refactor     #226      +/-   ##
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+ Coverage          74.89%   78.34%   +3.44%     
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  Files                 30       30              
  Lines               1665     1690      +25     
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+ Hits                1247     1324      +77     
+ Misses               418      366      -52