Sync the WCS helpers with photutils and speed up the region sky conversions

[larrybradley]

This syncs wcs_helpers.py with the photutils copy and applies the accompanying fixes to the region to_sky and to_pixel methods.

• Finite differences are now central, which removes a bias of half the curvature of the distortion field.
• A gwcs transform is evaluated with its bounding box disabled, so regions near the array edge no longer get NaN pixel scales.
• The longitude and latitude axes are selected by component index, so a latitude-first WCS converts correctly.
• The Jacobian evaluates the WCS once for all positions through the low-level interface, and the annuli convert both shapes with one evaluation.
• API change: the circular regions use the geometric mean of the Jacobian singular values as the isotropic pixel scale for every WCS.