adding positions to a database

[Sopel97]

Currently it's possible to add more pgn to an existing database but it's not exposed.

Doing this properly requires an "intelligent" scheme for when to merge files and which files to merge (for current two formats)

One way would be to remerge all files. Others could merge consecutive files with similar length. Overall there's no good way. It may require some hints from the user - in which case we would have many merging schemes and the user would choose one when making a request.