Adds Base Likelihoods, Maximum Likelihood Example

[BradyPlanden]

This PR adds:

• A base likelihood class
• Gaussian Likelihoods with known and unknown sigma values (initially based on Pints')
• Updates the optimisation class for likelihood definitions and min_iterations
• A probablility based cost function to return the negative log likelihoods
• A MLE example
• Tests for coverage

Closes #210.

[codecov]

Codecov Report

Attention: Patch coverage is 91.81818% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 94.29%. Comparing base (02ee2e2) to head (13309fb).
Report is 1 commits behind head on develop.

Files	Patch %	Lines
pybop/costs/_likelihoods.py	89.55%	7 Missing ⚠️
pybop/costs/fitting_costs.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #218      +/-   ##
===========================================
- Coverage    94.34%   94.29%   -0.05%     
===========================================
  Files           32       33       +1     
  Lines         1661     1753      +92     
===========================================
+ Hits          1567     1653      +86     
- Misses          94      100       +6     

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