Allow 'n' for time signatures to indicate none

[ahankinson]

In reference to this comment:

https://github.com/rism-digital/muscat-incipit-corrections/issues/42#issuecomment-1110006345

We may want an indication in the time signature field that no time signature was given in the original. Following the example of the key signature, we could adopt the n value (for "none"). It does not conflict with other values.

[ahankinson]

[] is needed for clefs and key signatures because without the supplied clef or accidentals, we cannot accurately represent the written or sounding pitch on a staff.

However, a time signature is not necessary to represent either written or sounding pitch.

So we will not permit n in the time signature field. If no time signature is present in the original, then this should be omitted.