Roll up of minor updates to wcs routines

- moved several calculation methods from AstroImage class to wcs module
- separated wcs library encapsulation to new wcsmod module
- updates to a few code routines to reflect these changes
- updated Pick module to calculate cdelt1/2 for starsize
  - fixes case where it gave an error for images without CDELTn keywords
Bug fix for Preferences plugin

- broke due to recent changes in wcs module