Evaluate usage of SourceModelContainer and remove it if possible

[stscijgbot-jp]

Issue JP-3725 was created on JIRA by Ned Molter:

SourceModelContainer is a data structure that's used to make a MultiExposureModel look like a ModelContainer so that those data could be processed through certain steps in calwebb_spec3, namely exp_to_source and extract_1d.  With the likely changes to ModelContainer in this epic, we need to take another look at whether SourceModelContainer is still useful and whether it should be modified or removed entirely from the pipeline.