change name and description of average edge to face

[jcapriot]

Fixes the averaging operator to actually do something that it describes. Before it was doing an odd operation that was only relevant to the boundary integrals, and even then only to coordinate aligned axes. Now it switches it to actually average things, and then changes it to the needed part for the boundary integral

[codecov]

Codecov Report

Merging #314 (838a15f) into main (bffe98a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #314   +/-   ##
=======================================
  Coverage   85.23%   85.23%           
=======================================
  Files          38       38           
  Lines       11301    11303    +2     
=======================================
+ Hits         9632     9634    +2     
  Misses       1669     1669           

Impacted Files	Coverage Δ
discretize/_extensions/tree_ext.pyx	84.44% <100.00%> (ø)
discretize/base/base_mesh.py	93.01% <100.00%> (ø)
discretize/operators/differential_operators.py	81.00% <100.00%> (+0.04%)	⬆️

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