Add AuxType to FEATS

[colinbatchelor]

This is to aid validation.

The possible uses of the copula are: https://gaelicgrammar.org/~gaelic/mediawiki/index.php/Copula

• Predicative: is mise Cailean 'I am Colin', is fliuch an oidhche 'the night is wet'
• Equative: Is e mise maor an fhearainn 'I am the landlord's understeward'
• Experiencing: is fheàrr leam 'I prefer'
• Cleft fronting a nominative: 's e Mòrag a rinn a bhanais 'It was Morag who had the wedding'
• Cleft fronting a predicate: 's ann an Ìle rugadh mi 'It was in Islay I was born'
  In this treebank we also mark the verb rach as an auxiliary where it forms the passive.
• Chaidh sin a dhearbhadh a-rithist anns na 1980an 'that was confirmed again in the 1980s'

[AngledLuffa]

Am curious, what kind of features would you be looking at here? We recently had an AuxType discussion for a Sindhi dataset where we eventually concluded it was likely to be redundant with the dependency graph:

UniversalDependencies/UD_Sindhi-Isra#9

but we could always revisit that if it looks like AuxType is being adopted elsewhere anyway

[colinbatchelor]

The passive vs. traditional copula case is indeed redundant so I'm only including that one for completeness.

However there are examples in the corpus of sentences that aren't obviously a cleft (the pronoun e is an augment and related to is with fixed) or a non-cleft sentence (the pronoun e is the head) without looking very carefully at the rest of the sentence, and sentences with the copula that don't fit any of the above patterns. Currently I'm thinking:

AuxType=Pred
AuxType=Eq
AuxType=Cleft
AuxType=Pass
AuxType=Exper

... open to suggestions for the last, maybe Psych or Quirky.

[colinbatchelor]

Collecting examples here: https://github.com/UniversalDependencies/UD_Scottish_Gaelic-ARCOSG/blob/dev/not-to-release/aux_examples.csv

[colinbatchelor]

On thinking about this in more detail and having gone through the non-clefting uses of the copula described in #43 I think a lot of what I want from a possible AuxType can be achieved by checking whether the copula has ExtPos=AUX and examining the next UD word. I'll leave this issue open for now but I am minded to not implement this.