Reconcile MARC with PAE2 requirements

[ahankinson]

Currently $o (time signature) in MARC21 says:

"Time signature or mensuration sign of the incipit. The time signature may be transcribed as a symbol (for mensural notation use “c”, “c.”, “o”, or ”o.”; or “c” for common time, “c/” for alla breve) and/or a number (“3", “2", “c3", etc.) or a fraction (“4/4", “12/8", etc.) or "nd" if marking is not present. Use of subfield $o is mandatory if subfield $p is present or subfield $2 contains either code "pe" or "da."" (emphasis added)

This is at odds with the current specification that says that time signature in PAE2 is optional and may be omitted. At the same time, it does not place a similar requirement for clefs ($g) whereas we do.

At some point in the ratification process we should try to reconcile these two things. There's a useful escape hatch that says it must be there for "pe" , so we can say that it's optional for "pe2", and likewise we can say clef is mandatory if $2 is pe2. But that will need to be sorted out.