Should `index` and `number` in global measure be required?

[GeorgeTailor]

Similar to #326 if the value is omitted, some default should be present, meaning that both index and number should be required in global measure from the specification point of view?

[adrianholovaty]

The docs specify the default behavior for both "index" and "number" if either of them is empty:

• "index" — The default value is 1 for the first measure in "global". For all other measures, the default value is the value of the previous measure plus 1.
• "number" — If omitted, its value is the same as "index".

With that in mind, it makes sense that neither of them is required, because sensible defaults are in place. In general, the philosophy of MNX is that we try not to encode information that's derivable or duplicate; this helps avoid ambiguous situations in a document.

Marking this as closed because I don't see anything that needs to change here.