References need enhancement

[raquelalegre]

At the moment references are listed in an array, children of lines, notes, etc.
For example, in belsunu there is this line in a translation:

1. Year 63, Ṭebetu (Month X), night of day 2:^1^

@note ^1^ A note to the translation.

The way it's being stored is in an array of references in the line object, and duplicated in an array of references in the note object.

We should come up with a way of:
a) not duplicating
b) linking the reference to both the note and the line
c) linking the object with the specific place it has been written. It could be an individual word, another part of the line (some random lemma), etc.
d) do a, b and c in a way that doesn't overcomplicate the serializer

[raquelalegre]

Eleanor said references should be linked to words. The lexer and serialisation should be changed accordingly.