Support grammatical number in concept model

[opoudjis]

I have no idea where this fits in to the glossarist universe, but grammatical number needs to be captured just like grammatical gender: we need to be able to indicate that a designation is singular or plural.

I am adding Number after Gender in the concept model. The conventional abbreviations are sg and pl.

See metanorma/metanorma-iso#554

[opoudjis]

The possible values are:

sg, pl, dual

Refer https://github.com/glossarist/concept-model/blob/master/images/Designations.png:

this is under GrammarInfo, as number: GrammarNumber[0..*]