Direct Input of Fourier Coefficients for Axis, Nvec, and Bvec in G-frame

Currently, to compute equilibria in the G-frame using GVEC, users must first provide the \zeta-grid values of the axis, Nvec, and Bvec, which GVEC then fits to Fourier coefficients. However, the CurveXYZFourierSymmetries class in SIMSOPT already represents spatial curves via Fourier coefficients. I propose allowing direct input of these Fourier coefficients to GVEC, bypassing the intermediate step of \zeta-grid value generation and fitting.

Additionally, GVEC could implement automated methods (e.g., parallel transport frame or rotation-minimizing frame) to compute Nvec and Bvec from the axis geometry. This would significantly streamline workflows for combined SIMSOPT+GVEC optimizations in the G-frame.