Populated Proskomma as source step

[mvahowe]

Right now, running a Proskomma query means

• adding a USFM document to Proskomma
• running the query
• deleting the document from Proskomma (so it's ready for next time, since the Proskomma instance is persistent)

Maybe we could treat Proskomma as a type of source (as an alternative to 'local' and 'http') that returns the Proskomma instance? Initially, we could provide a USFM document which would be loaded once. Subsequent queries would then be called on this populated instance, which is much faster than reloading and removing the data multiple times.