Handle partial verses

[mvahowe]

These are (sometimes) marked up with the \v tag (and sometimes marked up using \vp which means they are invisible to Paratext versification.) It doesn't seem that the current users of Proskomma are concerned about alignment using partial verses, but we should still render them.

I think that Proskomma will currently produce scopes something like verse/2b which may or may not work as expected downstream.

My proposal is to use a specific scope class for partial verses, and to treat them like vp.

[mvahowe]

Also need to do something with partial verse mappings in vrs files.