ENH: Initial codestubs for MergerTree to GenericIO
ENH: Distinguish between POSIX and MPI GenericIO
ENH: Update default parameters
ENH: Add WriteTree to DistributedHaloEvolutionTree
ENH: Add MergerTreeFileFormat ivar to MergerTree
ENH: Use supplied I/O strategy for the mergertree

Allocate GenericIO writer on the heap and specify the flavor
of I/O prescribed by the user.
ENH: Avoid separate write for the CRC

Allocate extra space to store the CRC for each variable to avoid
a separate write for the CRC.
ENH: Implement ForwardHaloTracker::WriteMergerTree
ENH: Remove MergerTreeFileName ivar
ENH: Propagate mergertree filename from config
ENH: Change halotracker to write GenericIO output