Reformat signal into a list

[NicolaCourtier]

• Make Dataset contain a dictionary
• Extend signal into a list of strings
• Align NLopt and SciPy optimisers with set_max_iterations
• Parameterise test_spm_optimisers and add test_multiple_signals
• Update Changelog

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a0d44e5) 93.03% compared to head (ca63085) 93.41%.
Report is 8 commits behind head on develop.

Files	Patch %	Lines
pybop/_problem.py	93.10%	2 Missing ⚠️
pybop/optimisers/nlopt_optimize.py	66.66%	1 Missing ⚠️
pybop/optimisers/scipy_optimisers.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #147      +/-   ##
===========================================
+ Coverage    93.03%   93.41%   +0.37%     
===========================================
  Files           34       34              
  Lines         1178     1199      +21     
===========================================
+ Hits          1096     1120      +24     
+ Misses          82       79       -3     

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[BradyPlanden]

Looks great Nicola! I've added some comments to take a look at, hopefully they are helpful. Overall, great contribution 👍

[NicolaCourtier]

Thanks very much for the review @BradyPlanden! It would be great if you could double check the 3 conversations above and either add more info or mark as resolved.