RFC: Change e_scale parameter and CatalogEntry.gal_scale from pixels to degrees

[kbarbary]

Nearly all the parameters of the model and CatalogEntry are given in world coordinates, independent of image pixel scale or WCS. For example, the position is given in (RA, Dec), not (x, y) pixel position. This makes sense because a single source may occur on multiple images, potentially with different pixel scales.

However, CatalogEntry.gal_scale seems to be an exception, being given in pixels. This (and the e_scale model parameter) should be changed to units of degrees.

Thoughts?