Allow data_factories to return hdulist objects

[ChrisBeaumont]

This would make it easier to set up glue coordinates

[astrofrog]

@ChrisBeaumont - could you explain a bit more what you were thinking here?

[ChrisBeaumont]

Right now, functions decorated with @data_factory can return a bunch of objects besides Glue Data instances, and Glue tries to package these appropriately (for example, it can accept astropy tables or pandas dataframes). That logic is handled by qglue.parse_data https://github.com/glue-viz/glue/blob/master/glue/core/data_factories.py#L249

It would be nice if parse_data knew what to do with HDUList objects, including packing the wcs into a Glue coordinate object and adding it to the dataset. Such logic is implemented in https://github.com/glue-viz/glue/blob/master/glue/core/data_factories.py#L347, but a user can't easily take advantage of it in a custom data loader

[astrofrog]

@ChrisBeaumont - ok, thanks for the clarification, I can look into it soon.

[ChrisBeaumont]

@astrofrog I can implement this