add test_frontal_area_index, use math.inf and math.nan

[erexer]

closes #44

[codecov]

Codecov Report

Merging #77 (ee74c8a) into feature/nodes (821d4e6) will increase coverage by 1.59%.
Report is 11 commits behind head on feature/nodes.
The diff coverage is n/a.

❗ Current head ee74c8a differs from pull request most recent head 6523e14. Consider uploading reports for the commit 6523e14 to get more accurate results

@@                Coverage Diff                @@
##           feature/nodes      #77      +/-   ##
=================================================
+ Coverage          66.06%   67.66%   +1.59%     
=================================================
  Files                  4        4              
  Lines                501      501              
=================================================
+ Hits                 331      339       +8     
+ Misses               170      162       -8     

see 1 file with indirect coverage changes

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[levisweetbreu]

This is good, but frontal_area_index() has changed #79

[erexer]

closes #79