Adapt prior and param-init to forward map

[LucaMantani]

This PR adapts the param initialisation to the existence of the forward map and consequently the possible presence of extra parameters.

NOTE: the node doing the parameter initialisation is currently called "pdf_initial_parameters" but now this is a bit of a misnomer since it can be a list of more parameters than just the pdf ones. Maybe we can consider renaming things? Alternatively, we could think of separating into two initialisation, pdf and extra, and then concatenate them.

Extra parameters can be controlled from the runcard in a totally symmetric way to pdf parameters, in addition to specifying their existence in the list extra_param_names:

prior_settings:
  prior_distribution: uniform_parameter_prior
  prior_distribution_specs:
    bounds:
      c1: [-1, 1]
      c2: [-3, 3]

param_initialiser_settings:
    type: uniform
    bounds:
        c1: [-1, 1]
        c2: [-3, 3]

extra_param_names: [c1, c2]

[codacy-production]

Up to standards ✅

🟢 Issues 5 high · 1 medium · 2 minor

Results:
8 new issues

Category	Results
UnusedCode	1 medium 1 minor
ErrorProne	3 high
Security	2 high
CodeStyle	1 minor

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.36%. Comparing base (4065a3a) to head (23ca96d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
- Coverage   96.36%   96.36%   -0.01%     
==========================================
  Files          32       32              
  Lines        1678     1677       -1     
==========================================
- Hits         1617     1616       -1     
  Misses         61       61              

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