Improve Localization and Extension Fitting

[woodmd]

This PR makes many improvements in the extension and localize methods:

• Improved method for fitting localization surface implemented in fit_parabola. The fit is now performed over pixels less than a certain pixel distance OR with delta-loglikelihood values larger than a threshold.
• Unify method for updating position or extension of a source. This is now handled through the set_source_morphology method which can be used to modify the morphology of a source in-place.
• Add fit_position option to extension which can be used to perform a joint fit of position and angular size.
• Preserve free/fixed state of parameters when returning from extension and localize.