Add taging scheme for morphological underspecification

implement an optional tagging scheme able to encode morphological ambiguity stemming from underspecification.

As per page 16 of my thesis Visualising Overlooked Morphosyntactic Alternatives

"Recent work by James Tauber on the MorphGNT dataset illustrates a shift toward more granular and transparent annotation of certain types of morphological ambiguity. He proposes introducing tags that explicitly mark morphological underspecification (for example, labelling a form as “non-feminine” when it could be either masculine or neuter). [47] An example of such a tagging schema appears in his parsing of the SBLGNT, where the article τῶν is tagged both as GPN (indicating functional interpretation) and GPX (indicating grammatical form), with POS information recorded separately. [48] This distinction enables researchers to differentiate underspecification from other forms of ambiguity in a principled way."

[47] James Tauber, “New Draft Morphological Tags for MorphGNT,” accessed 15 July, 2025, https://jktauber.com/2018/02/03/new-draft-morphological-tags-morphgnt/.
[48] See for example this line showing the annotation of τῶν in Jude: James Tauber, “sblgnt/86-Jud-morphgnt.txt,” accessed 15 July, 2025, https://github.com/morphgnt/sblgnt/blob/3b5cbdf92cb5e9bdb48a48026bcc0a08f885dd2c/86-Jud-morphgnt.txt#L290.