Fixing jet accessor bug in derivations

Following !32610 (merged) there was a crash in derivation making:

AODtoDAOD 22:22:47 xAOD::Jet_v1 ERROR Unknown attribute (56) requested.
(Did you ask the wrong type ?)

This turns out to be because the accessors are more complicated than I realized -- we need both an enum and an addition to the map. Now that's done, so we should be good to go again. I hope the jet folks will yell if this is going to cause any problems!

Thanks @amete for the report, and @elmsheus for reporting this earlier (he let me get away with claiming it was a partial build problem, which it turns out it was not... sigh).

FYI: @elmsheus , @jcatmore , @mswiatlo , @maklein , @schae , @eschanet