Make AstroImage respect set_ioClass and set_wcsClass methods.
Make AstroImage respect set_ioClass and set_wcsClass methods.

The `__init__` method uses the class attributes `ioClass` and `wcsClass`
in the method definition for keyword defaults.  The class methods
`set_ioClass` and `set_wcsClass` change their resepective class
attributes, but the `__init__` method function call is not updated to
reflect the change.  Solution: set the `__init__` keywords `ioclass` and
`wcsclass` to `None`, test these for `None` first, then test
`self.ioClass` and `self.wcsClass` before falling back on the defaults.

ejeschke/ginga,naojsoft/ginga,naojsoft/ginga,ejeschke/ginga,ejeschke/ginga,pllim/ginga,naojsoft/ginga,pllim/ginga,pllim/ginga