[VMEC,SurfaceRZFourier] fix inconsistency regarding mpol

[jons-pf]

VMEC only ever uses poloidal modes up to m=mpol-1 with mpol being specified in the VMEC input. SurfaceRZFourier calls their poloidal Fourier resolution also mpol, but includes poloidal modes including up to m=mpol. This leads to inconsistencies when converting between VMEC input/output files and SurfaceRZFourier. This PR addresses this inconsistency.