ENH: Merger-tree diagnostics for a given interval

Handle merger-tree diagnostics for a given interval in
ParalelHaloMergerTree -- instead of HaloMergerTreeKernel --
which also computes the total count of each merger-tree event
across all ranks.
ENH: Use simulation timestep

Changed code such that it registers halos using the simulation
timestep and improved the progress reporting such that it is
not off-by-one
WIP: Separate HaloExchange code