Support starting a Dataset process

Requires #1547 (closed), #1549 (closed)

Step 6 of https://redmine.teklia.com/issues/3653: StartProcess must now support starting both Workers and Dataset processes.

A Dataset process may only be started if there is at least one dataset on the process, and if at least one dataset on that process is on the corpus of the process. No validation on dataset states is necessary; that will be the responsibility of each worker.