Consider making nodes public

[braingram]

@schlafly asked about saving a datamodel to an ASDF file. The following does not work:

>> import roman_datamodels.datamodels as rdm
>> m = rdm.ImageModel.create_fake_data()
>> import asdf
>> af = asdf.AsdfFile({"foo": m})
>> af.validate()
ValidationError: <roman_datamodels.datamodels._datamodels.ImageModel object at 0x1091d78f0> is not of type 'object'

Failed validating 'type' in schema:

and instead the saving the node is required:

>> af = asdf.AsdfFile({"foo": m._instance})
>> af.validate()

However _instance is not public.

This seems like a useful pattern to support with public API.

[PaulHuwe]

Agree with implementing a public API.