Added new function to casagui/utils/__init__.py, __convert_masks that…

… is used to convert the input mask discitonary into an astropy region in either pixel or world cooridates. Currently supported mask shapes are 'rect' and 'polygon'. For mor info see: https://astropy-regions.readthedocs.io/en/stable/shapes.html. In the future, this function needs to support return of 'crtf' format for use with tclean. Currently this is nort supported due to a bug in the pixel version of astropy regions (astropy/regions#471). A bug ticket has been opened.