ENH: Query memory usage from merger-tree

Added functionality to query memory usage from merger-tree.
BUGFIX: Fix erroneous asserts
BUGFIX: Guard for 1st timestep
BUGFIX: Specify NULL transform for halo exchange
ENH: Method to return bytesize for halo metadata
ENH: Add method to compute total memory of tree
ENH: Total number of bytes/timestep in-core

Add memory profiling to coun total number of bytes per timestep of
memory that is in-core.