SymmetricCircuits - CoilCurrentEqualityConstraint needed

[oliverfunk]

Description of issue / requirement to address

Related to #3341, a new type of constraint should be implemented that constrains the currents of various coils to be equal, or equal to a fraction of another coil.

This could be used in cases where the work done in #3341 causes issues or could be useful in testing/comparing SymmetricCircuits results to results from this constraint, which take different approachs to solving a similar problem.

This constraint, however, would not reduce the size of the optimisation state vector, which although is less optimal (and less mathematically pure), may be more reliable or more realistic (as coil currents can never be truly equal, physically).

Proposed solution

Alternative solutions

Additional context