ModelLibrary does not read exposure types from input models correctly

[emolter]

When AbstractModelLibrary attempts to classify input models by their exposure type, it checks for the attribute "exptype" in model.meta here. However, at least for JWST data, the schema expects the exposure type to live in model.meta.asn (see here), and this leads to failures like the one in this issue.

A few other lines in AbstractModelLibrary may also be affected, here and here.