Allow single-number time signatures

[craigsapp]

Here is an example of 4/4 meter displayed as "4":

https://polishscores.org/?id=19xx:88 https://polish.musicsources.pl/en/lokalizacje/galeria/druki-muzyczne/16121/2#nomenu

Allow for *met(4) to display a single centered number on the staff for the time signature.

Test data:

**kern
*M4/4
*met(4)
=
1c
==
*-

Current rendering:

View in VHV

This issue will basically extend the currently allowed *met(3) to all integers:

[craigsapp]

Implemented with commit https://github.com/rism-digital/verovio/commit/e6719279b53253e6dc5a1c91141833b3120e3695

Rendering of test data: