WIP: Cosmo2GenericIO
ENH: Add MergerTreeEvent enum
ENH: Add MergerTreeFileFormat enum
ENH: MergerTree I/O

Implemented MergerTree I/O in DistributedHaloEvolutionTree.

Both DIY I/O and Generic I/O are supported. User can control which
format via a MergerTreeFileFormat ivar that is propagated through
the configuration file. Further, DistributedHaloEvolutionTree,
implements logic for round-robin assignment of blocks in the file
to each rank.