Transform distributions

[NicolaCourtier]

Description

The implementation of parameters, distributions and transformations is over-complicated. The aim here is to simplify the definitions and interactions between these classes to improve maintainability. A new UnboundedDistribution is created to define the behaviour of a parameter without a statistical distribution. The get_transformed_distribution functionality is added to both univariate and multivariate distributions.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #).

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: $ pre-commit run or $ nox -s pre-commit (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: nox -s tests
• The documentation builds: nox -s doctest
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

❌ Patch coverage is 89.56229% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.36%. Comparing base (4628a65) to head (6f11595).
⚠️ Report is 48 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/parameters/distributions.py	88.18%	15 Missing ⚠️
pybop/parameters/multivariate_distributions.py	85.36%	12 Missing ⚠️
pybop/parameters/parameter.py	94.66%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #925      +/-   ##
===========================================
+ Coverage    89.41%   92.36%   +2.94%     
===========================================
  Files           63       73      +10     
  Lines         4857     5668     +811     
===========================================
+ Hits          4343     5235     +892     
+ Misses         514      433      -81     

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