Support partial (but unambiguous) butler lookups for pfsObject files

It's a bit of a nuisance to work with pfsObject files as the butler doesn't handle finding them without being given nVisit and pfsVisitHash, even when there's only one possible match.

Please fix this.  Even when there is more than one match, also support passing in a desired list of visits, e.g. butler.get("pfsObject", ..., visits=[110666, 19580205, ...])