Adopt standard treatment of temporal and unmarked nominals

[nschneid]

Until recently, corpora like EWT used the relations {nmod:tmod, nmod:npmod, obl:tmod, obl:npmod} for temporal and non-temporal NP adjuncts. This was recently revised to :unmarked for all NP adjuncts regardless of temporality. UniversalDependencies/docs/issues/1028

In Atis, however, :npmod is not used, and :tmod is applied on what appears to be a purely semantic basis, including to PPs (though it is not 100% consistent, as can be seen from the attachments of "tomorrow").

To bring this in line with the new English standard,

1. Current :tmod nominals (including PPs) should move the temporality information to a MISC feature, Temporal=Yes
2. nmod and obl nominals lacking a case dependent should (at least in most cases) add the :unmarked subtype