`dj_graph` prompt to spawn missing classes

[CBroz1]

Currently, AbstractGraph throws an error when it cannot find a table. It's possible that this table may not be in Spyglass. The error message could suggest users spawn missing classes in these cases.

schema = dj.Schema('that_database')
schema.spawn_missing_classes()

An alternate version would just spawn these classes on their behalf, but this could add unpredictably many classes to the namespace, potentially colliding with existing objects