Slight re-organization of some code in core/cluster.cpp

put "neighbor" class definitions in openbr_internal.h, export a couple
additional functions in openbr_plugin.h.
Add a function to get the comparison transformation used by an algorithm
Replace ProgressCounter's totalProgress var with a int64

since total progress can be a file size in bytes, this should really be a
64-bit number.

Soften output condition -- if FrameNumber metadata is not present, still display
progress