Fit in PCA basis

[giacomomagni]

In this PR we address:

• [x] fix the PCA computation subtracting the central values #34
• [ ] fix the PCA documentation
• [x] Add the possibility to fit in the PCA basis
• [x] Add some tests

[codecov[bot]]

Codecov Report

Merging #36 (5b893af) into smefit_ewpo (69f015e) will increase coverage by 3.00%. The diff coverage is 77.08%.

@@               Coverage Diff               @@
##           smefit_ewpo      #36      +/-   ##
===============================================
+ Coverage        40.59%   43.59%   +3.00%     
===============================================
  Files               27       27              
  Lines             2074     2156      +82     
===============================================
+ Hits               842      940      +98     
+ Misses            1232     1216      -16     

Flag	Coverage Δ
unittests	43.59% <77.08%> (+3.00%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/smefit/analyze/report.py	14.90% <0.00%> (-0.10%)	:arrow_down:
src/smefit/cli/__init__.py	0.00% <0.00%> (ø)
src/smefit/optimize/ns.py	61.11% <ø> (ø)
src/smefit/runner.py	47.57% <28.57%> (-1.39%)	:arrow_down:
src/smefit/coefficients.py	95.19% <91.66%> (+0.62%)	:arrow_up:
src/smefit/analyze/pca.py	68.58% <92.30%> (+10.64%)	:arrow_up:
src/smefit/basis_rotation.py	97.77% <100.00%> (+82.39%)	:arrow_up:
src/smefit/loader.py	92.30% <100.00%> (+0.51%)	:arrow_up:

[giacomomagni]

Hi @jacoterh, if this works and we are happy with the accuracy of the test I'm cool. Could you please extend a bit the discussion on the centering inside the docs :)