Add subfield $p to 690 for indicating the part or movement number

[lpugin]

Currently, references to single movements of a work in the 690 are to be added with a /x in 690$n. The guidelines read

If the source contains only one movement of a work, indicate the number of the movement after a slash.

See for example KV 425/3 in https://muscat.rism.info/admin/sources/450201735

The information would be better structured with two subfield, namely $n for the work number, and $p for the part or movement of the work.

This is an internal field and changing it would have no impact on the interoperability of the data. The display can be kept the same, but it would make links with works much better structured.

[xhero]

Do some statistics on this field

[alexandermarxen]

Just a reminder: Some catalogues such as GWV use the slash as a genuine component of the citation
Example: https://muscat.rism.info/admin/sources/450006566

[MFalletta]

And BPI we cite without slash acoording to the catalogue.
Examples: https://muscat.rism.info/admin/sources/990056940 (S=Sinfonia) or
https://muscat.rism.info/admin/sources/452506787 (Aria from the 1st act)