Handle VMEC Input Files Better

[dpanici]

Apparently in VMEC the boundary modes can be given with the following logic, which we currently do not support converting in DESC. Also, this could be useful to add as an input option in our own input files

rbc(-22:22,0) = 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
                 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
                 0.9752041013880218, -0.693363161259351, 0.3158097406419154,
                 -0.1336173987189171, 0.04250319143001195, -0.0017939240980503425,
                 -0.011073301883956194, 0.010402231681464624, -0.005115714919129545,
                 0.0001348021675985992, 0.002564506738780156, -0.0028903931958533633,
                 0.0018064368511084795, -0.00023521761858271873, -0.0007086499231920249,
                 0.0011111467874450785, -0.0005442993972098523, -1.2982368610783194e-05,
                 0.0007834476816723562, 2.2221180650775065e-06, -4.416136866781143e-05,
                 8.239232451572815e-05, -0.00010136354765774249