ENH: Add GlobalID to Halo data-structure

Added GlobalID ivar which will hold the unique ID of a halo for
all time-steps.
ENH: Remove halo-prefix sum strategy

Removed previous strategy that was relying on
the halo-prefix sum for re-numbering the nodes
in the merger-tree downstream. Instead, GlobaIDs,
i.e., IDs that are unique for each halo for all
timesteps will be computed and assigned directly.
ENH: Method to compute unique range per process

Added method that computes a unique range per process for
a number of items.
ENH: GlobalID numbering across all timesteps

Added code to assign global IDs to the provided halos at
a given timestep.
WIP: Add global IDs to Zombies