Allow multiple data collectors running simultaneously

With these changes it is possible to assign names to data collectors (DCs) and connect multiple DCs simultaneously to one run control instance:

- first DC connection is treated as default
- replaced if unnamed DC connects
- otherwise, named DCs are only assigned to producers named accordingly
- runnumber is verified between all DCs

This addresses issue eudaq/eudaq#25

Some outstanding issues/todos (before merging):
- [x] test with GUI
- [x] document
- [ ] test and verify data files with new MultiFileReader