add code to extract level set paths and lengths

[m-reuter]

Here we add code to:

1. compute the accumulated length of a level set, by finding intersecting points at edges in each triangle. This works for arbitrary level sets and can also loop over multiple level values.
2. extract a level set path for a single curve with start and end point and compute its length. This function also uses a helper function (wrapper around shortest_path) to sort undirected edges into a single path quickly.

The idea is to add this to the next minor release.

[codecov]

Codecov Report

Attention: Patch coverage is 7.75862% with 107 lines in your changes missing coverage. Please review.

Project coverage is 33.62%. Comparing base (2cbcee0) to head (5bb781d).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lapy/tria_mesh.py	7.07%	105 Missing ⚠️
lapy/diffgeo.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   34.89%   33.62%   -1.27%     
==========================================
  Files          14       14              
  Lines        2370     2483     +113     
  Branches      286      301      +15     
==========================================
+ Hits          827      835       +8     
- Misses       1458     1563     +105     
  Partials       85       85              

Flag	Coverage Δ
unittests	33.62% <7.75%> (-1.27%)	⬇️

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