Added the option for an initial individual

[jmafoster1]

Summary

Added the ability to specify an initial individual for HillClimberDiscovery, e.g. from a different causal discovery technique. I also moved max_iterations into the function arguments rather than the class object.

Checklist

Before you mark your PR as ready for review, please ensure you have completed the following.

• Added unit tests for new functionality
• Properly documented any new code
• Updated Sphinx documentation in docs/ and checked for updated configuration/runtime options
• Added a jupyter notebook tutorial in docs/source/tutorials for any new functionality
• Verified local docs build with sphinx-build -W source/ build

[github-actions]

🦙 MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
⚠️ PYTHON	black	30		1	1.29s
✅ PYTHON	pylint	30		0	6.05s

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.48%. Comparing base (189396e) to head (a264941).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #408   +/-   ##
=======================================
  Coverage   98.48%   98.48%           
=======================================
  Files          25       25           
  Lines        1517     1518    +1     
=======================================
+ Hits         1494     1495    +1     
  Misses         23       23           

Files with missing lines	Coverage Δ
causal_testing/discovery/hill_climber_discovery.py	100.00% <100.00%> (ø)
..._testing/estimation/linear_regression_estimator.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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