Add length multiplier to normals

[jo-mueller]

#280

This PR adds support to measure intensity on surfaces. If done interactively and in conjunction with other plugins (i.e. napari-segment-blobs-and-things-with-membranes), the time-slicing functionalities clash sometimes, but in itself, it works.

• Documentation added
• Tests added
• interactive use tests

[codecov]

Codecov Report

Merging #310 (b4fce26) into main (e5220f2) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   82.69%   82.82%   +0.13%     
==========================================
  Files          50       51       +1     
  Lines        9196     9266      +70     
==========================================
+ Hits         7605     7675      +70     
  Misses       1591     1591              

Impacted Files	Coverage Δ
src/napari_stress/_tests/test_reconstruction.py	100.00% <ø> (ø)
src/napari_stress/_measurements/__init__.py	100.00% <100.00%> (ø)
src/napari_stress/_measurements/intensity.py	100.00% <100.00%> (ø)
...c/napari_stress/_reconstruction/refine_surfaces.py	90.84% <100.00%> (+0.06%)	⬆️
src/napari_stress/_tests/test_measurements.py	97.59% <100.00%> (+0.17%)	⬆️
src/napari_stress/_tests/test_vectors.py	100.00% <100.00%> (ø)
src/napari_stress/_vectors.py	97.77% <100.00%> (-0.61%)	⬇️

... and 1 file with indirect coverage changes