Add helper functions to `_compute` registrations that return the grid needed for accurate computation on the requested eval grid

[dpanici]

helper would take in the object and the desired eval grid and return a grid that can accurately compute the quantity on that grid for that resolution.

[f0uriest]

as an example, for computing elongation:

def _helper(eval_grid, eq):
    return LinearGrid(rho=1, M=eq.M_grid, zeta=eval_grid.nodes[eval_grid.unique_zeta_idx,2])