Example of `FixSumCoilCurrents` that explains using it for symmetry/NFP symmetry for fixing net poloidal current to match equilibrium's

[dpanici]

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FixSumCoilCurrents will only fix the sum of the currents of the coils that have unique params in the coilset. An intended use for this objective is to fix currents in coils to sum to the amount 2*pi *G/mu_0 dictated by the equilibrium and Ampere's law #1111 . However for a CoilSet with say NFP symmetry, this objective will only be summing up over a single field period, and so it will have to sum to 2*pi *G/mu_0 / NFP instead. If the coilset has stellarator symmetry as well, it should target 2*pi *G/mu_0 / NFP/ 2

This does not really need to be in the docstring but maybe in a future update to the coil tutorial we could use this for one of the constraints

[dpanici]

Put in docstring of FixSumCoilCurrents