Add point2index functionality for tensor_mesh

[jcapriot]

Adds point2index functionality for finding which cell contains a point

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.65%. Comparing base (8f94934) to head (11f4c2a).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   86.61%   86.65%   +0.04%     
==========================================
  Files          91       91              
  Lines       18710    18769      +59     
  Branches     2111     2119       +8     
==========================================
+ Hits        16205    16264      +59     
  Misses       1820     1820              
  Partials      685      685              

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