Private metadata attributes are silently lost

[braingram]

>> from roman_datamodels import maker_utils as mk, datamodels as dm
>> m = mk.mk_datamodel(dm.ImageModel)
>> m.meta._foo = 1  # no error
>> m.meta._foo
AttributeError: No attribute _foo

I think this is in part due to _foo getting set on an ephemeral DNode instance.

>> id(m.meta)
4814979664
>> id(m.meta)
5611666880

Which leads to this fun line of code:

>> m.meta is m.meta
False

Saving a reference to and re-using the meta retains _foo:

>> mm = m.meta
>> mm._foo = 1
>> mm._foo
1

I'm not sure if this is a bug but I was surprised by it.

[WilliamJamieson]

This is expected behavior https://github.com/spacetelescope/roman_datamodels/blob/main/src/roman_datamodels/stnode/_node.py#L214-L215.

Really, we should not be allowing setting of private attributes, however this might break the node internally. I'll need to investigate this.