Refactor PP5 comparison handling

Introduce PP5GalleryTransform, a transform that compares incoming templates
against a fixed gallery (thereby avoiding the cost of repeatedly setting up
pp5 galleries for the same templates).

Modify AlgorithmCore to support supplying Transforms to the right of the
: in an algorithm string. If a distance cannot be created (testing if
defaultDistance is returned from Distance::make), store the comparison
part as a string, and use it to make a replacement for GalleryCompareTransfrom
in calls to AlgorithmCore::compare

This commit essentially disables DefaultDistance (which could previously
be used as a shorthand for DistDistance with some specific metric),
instead leaving DefaultDistance as a non-functional distance.
I guess I don't have a specific suggestion. Just a thought that it would be nice if we could move code for the special handling of DefaultDistance into DefaultDistance itself and avoid the conditional check for DefaultDistance in currently in AlgorithmCore. This may not be possible though, not sure.