Added fixed aperture flux target measure

[thuiop]

Closes #342

New target measure in the metrics module, for measuring the flux using a fixed circular aperture with sep.

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Codecov Report

Base: 88.54% // Head: 87.89% // Decreases project coverage by -0.65% :warning:

Coverage data is based on head (4c4ec78) compared to base (29e6564). Patch coverage: 36.84% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #373      +/-   ##
===========================================
- Coverage    88.54%   87.89%   -0.66%     
===========================================
  Files           11       11              
  Lines         1563     1578      +15     
===========================================
+ Hits          1384     1387       +3     
- Misses         179      191      +12     

Flag	Coverage Δ
unittests	87.89% <36.84%> (-0.66%)	:arrow_down:

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Impacted Files	Coverage Δ
btk/metrics.py	88.74% <36.84%> (-3.49%)	:arrow_down:

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[ismael-mendoza]

1. fix pixel scale
2. compare apeture flux photometry (sextractor vs scarlet), scatter plot
3. documentation update
4. change labels of figures
5. (Maybe) true locations conditioned

[thuiop]

I fixed the pixel scale issue and updated the doc ; I am merging this now and will open another PR for the plot (there are a few things I need to change, and I'll create a new notebook for reproducibility).