Add mechanism for introspecting progress of a `ProtocolDAG` as it is running

[dotsdl]

Currently, users can only tell what Tasks are running by way of their status. For running Tasks, no further information is available until the Task completes or errors out.

We should add a mechanism by which the compute services send updates to their CLAIMS relationship the number of ProtocolUnits completed, as well as the total number of ProtocolUnits in the ProtocolDAG. That will enable us to add a simple AlchemiscaleClient method to report a running Task's progress.

This update could be achieved via a separate thread, very similarly to how heartbeats are currently performed.

Once a system like that proposed in #180 is implemented, more detailed information could be given on the progress of individual ProtocolUnits within the ProtocolDAG beyond simply counts.