Multivariate parameters

[SarahRo]

Description

Introduce MultivariateParameter (subclass of pybop.Parameter) instead of MultivariateParameters (subclass of pybop.Parameters) and edit pybop.Parameters to allow for multivariate parameters.

The MultivariateParameter class is almost identical with the Parameter class but it additionally requires the name of the paramater that holds the MultivariateDistribution. This is a little bit hacky but allows the parameters to be passed without already being collected into one object.

A check_multivariate method is added to pybop.Parameters() that checks whether a MultivariateParameter is present and if so whether all parameters are of type MultivariateParameter and point to the same distribution. This method is called in the constructor, the set method and the add method. The call to the method can be disabled in the add and set method by setting the parameter check_multivariate to false. This is, for example, used in the constructor in order to only call the method once all parameters have been added.

The edits to the Parameters class are also a little bit hacky. However, this approach means the Problem and Simulator classes do not explicitly need to be aware whether they are dealing with mulitvariate parameters.

An alternative approach would be to discard the changes to pybop.Parameters, reintroduce the MultivariateParameters class and construct the MulitvariateParameters class in pybop.pybamm.Simulator if any parameters of type MultivariateParameter are provided to the constructor as part of the ParameterValues object. This means that Problem and MetaProblem need to distinguish between MultivariateParameters and Parameters as well (this would only require minor edits).

Fixes #856

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 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.81%. Comparing base (4628a65) to head (9a8feca).
⚠️ Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
pybop/parameters/parameter.py	97.77%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #862      +/-   ##
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+ Coverage    89.41%   90.81%   +1.39%     
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  Files           63       65       +2     
  Lines         4857     5169     +312     
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+ Hits          4343     4694     +351     
+ Misses         514      475      -39     

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