Need to make it possible for a user to set the header extension in the reference image

[galaxyumi]

In 'make_ast_xy_list.pick_positions_from_map', refimage_hdu is set to be 1. I ran into an error when one of my reference image only has the zero-th extension. We probably want to make it possible for a user to set the WCS header extension of their reference image in a datamodel.py or a new datamodel class.