Local_dc work on issue #201.
- Added a check that each non-iop model must have the same cycle and second as the iop model, not just the same cycle.
- When checking for data from the iop the system checks for progression, so we know we are seeing a live iop.
- Minor cleanups (removing an unused variable, replacing a numeric constant with a symbolic constant)
- Added a test to check that 'old' dcus are not sent on.
Closes #201 (closed)