add XPOS for verbs and numbers

[nschneid]

• Knowing the form of the verb is important for detecting clause-level constructions like passives
• Knowing the difference between numbers and other N/D words can be useful, and the distinction is made in PTB and UD (including the nummod relation distinct from det)
• Rather than implement a full set of morphosyntactic features, an incremental step is to import fine-grained POS tags. UD has XPOS tags which for English follow the PTB tagset.
  • Can go directly in .cgel file with :xpos feature, alongside the existing lexical categories
• For now, limit to CD (number), MD (modal aux), and the following for all other verbs: VB (plain form), VBD (preterite), VBG (gerund-participial form), VBN (past participle), VBP (present tense except 3sg), VBZ (present 3sg)
  • For verbs, CGEL makes some finer-grained distinctions, like modal present vs. preterite, and incorporates negation clitics. And the tags collapse some of the paradigm cells for be. But these further distinctions are groupings of closed-class lexemes, so it isn't strictly necessary to have finer-grained tags.
  • Importing all XPOS tags might cause confusion as there are significant boundary differences for categories like P and D in CGEL versus other schemes (as we explored here)

[nschneid]

merged in #93

[nschneid]

We also have :xpos "LS" for list item markers.