Separate forward map

[LucaMantani]

This PR aims at separating the definition of the forward map from the pdf_model, granting us further flexibility to implement generalisations such as the addition of nuisance parameters or eft Wilson coefficients.

The main changes are:

• remove pred_and_pdf function
• Definition of an abstract class ForwardMap, which requires always the implementation of a __call__method with signature (fk_arrays, params) -> predictions, pdf
• The implementation of a particular case, which is the one we are currently using that does only PDF fitting.
• Params are all the parameters of interest, and the first n_pdf_params are always assumed to be the pdf ones.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.36%. Comparing base (bad34b4) to head (177898a).
⚠️ Report is 55 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #428      +/-   ##
==========================================
+ Coverage   96.13%   96.36%   +0.23%     
==========================================
  Files          31       32       +1     
  Lines        1654     1678      +24     
==========================================
+ Hits         1590     1617      +27     
+ Misses         64       61       -3     

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[codacy-production]

Up to standards ✅

🟢 Issues 35 high · 18 minor

Results:
53 new issues

Category	Results
UnusedCode	1 minor
Documentation	17 minor
ErrorProne	5 high
Security	30 high

View in Codacy

🟢 Metrics 43 complexity · 3 duplication

Metric	Results
Complexity	43
Duplication	3

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