Make `FixBoundaryR` work for `FourierRZToroidalSurface` as well

[dpanici]

Right now fixing eq surface and a fourierRZtoroidalSurface has two different syntax (I know that the eq surface could be fixed with FixParameters as well by passing in "Rb_lmn", but this is hte usual and documented way):

    # Fix the M=0 N=0 of the eq and surface
    FixBoundaryR(eq, modes=[[0, 0, 0]]),
    FixParameters(surf, {"R_lmn": [idx_R00_mode_surf]}),

it would be nice if FixBoundaryR also worked for surfaces