ENH: Add Halo::GetNumberOfParticles
ENH: Add Get/Set macro for MergerTreeThreshold
ENH: Clear halo particle Ids for Evolution tree

To save memory, once the halo is copied to the halo
evolution tree, the halo particle IDs are cleared out.
ENH: Add user-supplied parameters to Halo tracker

Added methods to set parameters for the merger-tree
and halo-evolution tree that are managed internally.
Further, added ForwardHaloTracker::Initialize method
to properly initialize these data-structure, i.e.,
supply the MPI communicator and other parameters etc.