datamodels.open does not work on a datamodel

[zonca]

The workaround is this commit:

701a431

this is triggered by get_reference_file, see below for more details. It seems like JWST has no issues passing a Data model to get_reference_file, however, it seems we can only pass a string. We need to understand where this issue is coming from.

• 701a431 I see how this stems from an assumption that get_reference_file expects a filename, not a DataModel. Should we override get_reference_file to accept both?

There is something I do not understand here, maybe you can track it down,
see here:
https://github.com/spacetelescope/jwst/blob/10501f22def0460b15df65cd5261601c6a71d7cf/jwst/dark_current/dark_current_step.py#L32

they are doing the same, feeding the input model to get_reference_file, why does it work for them and not for us?
I think that at some point in the class hierarchy we are using different classes but have not understood where.

Originally posted by @zonca in #58 (comment)