Improve root finding accuracy on low res grids

[unalmis]

After reducing resolution of Quadrature grid in draft pull request, the map_coordinates test is failing because this decreases the number of radial knots used to construct a cubic spline of iota, which deteriorates the 3d root finding used by default in map_coordinates.

Given that iota is the main ingredient in all these coordinate mappings, the solution is to update map_coordinates to also compute iota_r and feed that into interpax.interp1d with the undocumented kwarg fx=iota_r to compute Hermite spline.

Originally posted by @unalmis in #1188 (comment)