Add check to CMAES and make XNES default optimiser

[NicolaCourtier]

Description

Only CMAES appears to be restricted to optimising multiple parameters at a time. This suggests that we should change the default optimiser away from CMAES.

Issue reference

Fixes #91.

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[NicolaCourtier]

I'm open to discussion on which optimiser to set as default, but the restriction on CMAES to multiple parameter optimisation problems only seems to rule it out from being the most user-friendly option!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.14%. Comparing base (133af44) to head (34d2e24).
Report is 3 commits behind head on develop.

❗ Current head 34d2e24 differs from pull request most recent head 2359be2. Consider uploading reports for the commit 2359be2 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #253      +/-   ##
===========================================
- Coverage    95.33%   95.14%   -0.20%     
===========================================
  Files           36       36              
  Lines         1971     1996      +25     
===========================================
+ Hits          1879     1899      +20     
- Misses          92       97       +5     

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[BradyPlanden]

Thanks for the addition @NicolaCourtier. I agree, CMAES is probably not the most robust default. In the future, we may want differing defaults dependant on the dimensionality of the problem.

I took a quick look at the benchmarks and XNES appears to be the fastest pints optimiser for the two parameter problem, so another checkmark moving forward.

I've added a small tweak to be more explicit in the ValueError, happy to merge either way.