Multivariate parameters

[SarahRo]

Description

Introduce MultivariateParameter (subclass of pybop.Parameter) instead of MultivariateParameters (subclass of pybop.Parameters)

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 90.73%. Comparing base (4628a65) to head (3b196b5).
⚠️ Report is 11 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      +/-   ##
===========================================
+ Coverage    89.41%   90.73%   +1.31%     
===========================================
  Files           63       66       +3     
  Lines         4857     5224     +367     
===========================================
+ Hits          4343     4740     +397     
+ Misses         514      484      -30     

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