Greek letters as symbols a11y update

[StephDriver]

Greek letters are used in articles, sometimes in the text to refer to items (e.g. mathematics or linguistics) and often for units (e.g. micrograms, nanometres). These need to be marked-up for screenreaders appropriate to their context (e.g. the former might be 'mu' but the later might be 'micrograms').

We need to consider how these are presented in the JATS in order to interpret them the way the author expects and intends.