Adds Pints' optimisers

[BradyPlanden]

Closes #116. Adds the remaining Pints' optimiser methods, and updates examples.

I've left _nelder-mead.py and _cmaes_bare.py out of this PR, as we have access to the former via SciPy and I don't believe we need two CMAES methods for the latter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (83ec94b) 93.75% compared to head (eee379d) 90.57%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #117      +/-   ##
===========================================
- Coverage    93.75%   90.57%   -3.19%     
===========================================
  Files           29       18      -11     
  Lines          993      647     -346     
===========================================
- Hits           931      586     -345     
+ Misses          62       61       -1     

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[BradyPlanden]

This is now ready for review. In this PR, I've forked one of the example scripts and updated it to include the different optimiser methods we have. We may want to consider moving these to a separate directory, such as examples/scripts/optimisers.

Unfortunately, the tests have regressed, but this is to be expected as I have removed the `tests/' directory from the CodeCov bot. The patch percentage is good for the merge, but we will need to add more coverage in the future.

[NicolaCourtier]

Excellent work on adding the optimisers, thanks @BradyPlanden !