Fix occasional optimiser test failure

[CoronelBuendia]

Linked Issues

Closes #1652

Description

Fixes the optimiser test.

I went through the whole damn thing, checked the gradient calculations of the toy problem, and went pretty deep into what we have. I've narrowed it down to two things:

• Invalid starting solution does not guarantee convergence (fixed)
• Likely something wrong with the SLSQP implementation in NLOpt (unconfirmed)
  • Partly related to the strange problem formulation I had chosen in which 3 of the variables were basically dummy equality constraints that didn't feed into the objective function, but it seems that very occasionally SLSQP's Hessian calculation (again probably) causes a starting solution of zeros to be passed into the objective function repeatedly until the number of iterations is reached.
  • This is probably a very strange edge case in the SLSQP algorithm, and the problem formulation was not a good one in any case.
  • I fixed this by changing the objective function so that it used all variables and kept all the inequality and equality constraints as before.

Interface Changes

N/A

Checklist

I confirm that I have completed the following checks:

• Tests run locally and pass pytest tests --reactor
• Code quality checks run locally and pass flake8 and black .
• Documentation built locally and checked sphinx-build -W documentation/source documentation/build

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #1651 (7745a4b) into develop (ebec2de) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1651   +/-   ##
========================================
  Coverage    74.65%   74.65%           
========================================
  Files          162      162           
  Lines        19213    19213           
========================================
+ Hits         14343    14344    +1     
+ Misses        4870     4869    -1     

Impacted Files	Coverage Δ
bluemira/equilibria/solve.py	68.22% <0.00%> (+0.52%)	⬆️

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[github-actions]

⚠️ Warning Report

Found 0 new warnings, 0 fixed warnings. 🎉