Add OffsetTracker field.  TODO: actually use.
Add OffsetTracker field.  TODO: actually use.

git-svn-id: https://svn.sable.mcgill.ca/mclab/Project@518 89b902b4-d83f-0410-b942-89f7a419ffdb

Migrated from Sable/mclab@646622fe59d5e22315c805c6b0a46dec03925d31

Sable/mclab-core,Sable/mclab-core,Sable/mclab-core,Sable/mclab-core,Sable/mclab-core,Sable/mclab-core,Sable/mclab-core,Sable/mclab-core