Do not guess pixel area when transforming

[maxnoe]

Instead of using the guess pixel area method, we now calculate a scale from the transformed unit vectors.

This is both faster and also works well for cameras with non-uniform pixel sizes or gaps between pixels:

Before

After

[codecov]

Codecov Report

Merging #1794 (81612d0) into master (5ff5401) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1794      +/-   ##
==========================================
+ Coverage   91.83%   91.95%   +0.11%     
==========================================
  Files         190      190              
  Lines       15241    15244       +3     
==========================================
+ Hits        13996    14017      +21     
+ Misses       1245     1227      -18     

Impacted Files	Coverage Δ
ctapipe/instrument/camera/geometry.py	91.51% <100.00%> (+0.07%)	⬆️
ctapipe/tools/camdemo.py	76.13% <0.00%> (+20.45%)	⬆️

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[LukasNickel]

Sorry, didnt see the failing test, thought that was codacy.

[kosack]

I suspect the failed test is some random fluctuation... but better check