Multi-start functionality for optimisers

[BradyPlanden]

Description

Adds multi-start functionality to Pints' and SciPy optimisers.

ToDo:

• Add integration and unit tests
• Verify plotting and updates to optimiser log for multi-start storage

Issue reference

Fixes #438

Review

Before you mark your PR as ready for review, please ensure that you've considered the following:

• Updated the CHANGELOG.md in reverse chronological order (newest at the top) with a concise description of the changes, including the PR number.
• Noted any breaking changes, including details on how it might impact existing functionality.

Type of change

• New Feature: A non-breaking change that adds new functionality.
• Optimization: A code change that improves performance.
• Examples: A change to existing or additional examples.
• Bug Fix: A non-breaking change that addresses an issue.
• Documentation: Updates to documentation or new documentation for new features.
• Refactoring: Non-functional changes that improve the codebase.
• Style: Non-functional changes related to code style (formatting, naming, etc).
• Testing: Additional tests to improve coverage or confirm functionality.
• Other: (Insert description of change)

Key checklist:

• 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 unit tests pass: $ nox -s tests
• The documentation builds: $ nox -s doctest

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

Further checks:

• Code is well-commented, especially in complex or unclear areas.
• Added tests that prove my fix is effective or that my feature works.
• Checked that coverage remains or improves, and added tests if necessary to maintain or increase coverage.

Thank you for contributing to our project! Your efforts help us to deliver great software.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.17%. Comparing base (c8b00e6) to head (8e7f8d6).
Report is 106 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #571      +/-   ##
===========================================
+ Coverage    99.08%   99.17%   +0.08%     
===========================================
  Files           52       56       +4     
  Lines         3605     3991     +386     
===========================================
+ Hits          3572     3958     +386     
  Misses          33       33              

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[martinjrobins]

looks great, just a question on the docstring below