DM-25692: Add gen3 formatter for Filter

[timj]

This allows an Exposure to be disassembled and assembled without
relying on adding something to metadata.

[kfindeisen]

There are some issues with the type handling in the new code (there's a mix of generic and highly type-specific references to the filter), but nothing that should require re-review.

[kfindeisen]

Please be more specific. We shouldn't be encouraging people to catch Exception.

[timj]

In the general sense of any formatter, we have no real idea what exceptions could be raised. It doesn't really matter either because all formatter exceptions are caught by datastore.

[kfindeisen]

This is a bit contradictory, and I'm not sure what to do about it. On the one hand, formal type safety requires that this be an object of any type. On the other hand, the method body explicitly assumes that inMemoryDataset is a Filter, and will raise an AttributeError (convert to TypeError?) for any other type.

[timj]

We haven't yet developed a convention for someone configuring their datastore such that the wrong python type turns up in the formatter.

[timj]

Thanks. Sorry about all the docstring problems. I copied them from YamlFormatter and forgot to update them.