ENH: Individual halo access in FOFHaloProperties

Added methods to access halo particle IDs, halo center
and halo individually for a given halo.
ENH: Better define IsComplete

Temporal halo information is complete if both current
and previous halo information objects are allocated and
if in both current and previous time-steps we have halos.
WIP: Parallel merger-tree

Working towards parallel, incremental merger-tree
construction. Currently, only stabs for the
communication.