Add porosity_threshold function to find percolation threshold

[jgostick]

This adds 2 functions:

• porosity_threshold erodes the void space until the void space no longer percolates, yielding the percolation threshold
• is_percolating simply answers the question if the domain percolates or not

[codecov]

Codecov Report

Attention: Patch coverage is 6.34921% with 59 lines in your changes missing coverage. Please review.

Project coverage is 67.2%. Comparing base (751232d) to head (3bc08c3).

Additional details and impacted files

@@           Coverage Diff           @@
##             dev   #1091     +/-   ##
=======================================
+ Coverage   66.7%   67.2%   +0.5%     
=======================================
  Files         43      42      -1     
  Lines       6149    6100     -49     
=======================================
+ Hits        4102    4105      +3     
+ Misses      2047    1995     -52     

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