Improve pixel <-> sky region conversions

[larrybradley]

This PR improves the pixel <-> sky region conversions, especially for WCS with distortions:

• For non-distorted astropy WCS objects, the pixel scale is computed
  from 1-pixel offsets along both the x and y axes, with the geometric
  mean taken as the local isotropic scale. The North angle is computed by
  offsetting in latitude by exactly this local pixel scale, ensuring the
  finite-difference derivative is evaluated at the appropriate angular
  scale.

• Sky-to-pixel and pixel-to-sky coordinate conversions now use the
  local WCS Jacobian matrix for WCS objects with distortions (e.g., SIP
  polynomial corrections) or non-astropy WCS objects (e.g., GWCS). This
  provides accurate directional scale factors along each axis of directed
  regions (ellipses, rectangles, and their annuli) under distorted WCS,
  rather than assuming an isotropic pixel scale.

Example
Regions originally defined in Sky coordinates in a non-distorted WCS image, then converted to pixel-based regions with a distorted WCS (an HST ACS/WFC FLC image whose WCS has both SIP and tabular distortions):

• Green: polygonized regions (using Add to_polygon method to region classes #649) -- taken as "truth". Polygon regions transform "exactly" (ignoring spherical projection)
• Red: conversions based on regions main (a single scale factor applied to all shape axes)
• White: conversions based on this PR -- which gives better results