Document the process scripts

Add doctrings for the main processing scripts and tell about their parameters: e.g.

- [ ] `erna.scripts.process_fact_data.py`
- [ ] `erna.scripts.process_fact_mc.py`
- [ ] `erna.scripts.process_fact_data_qsub.py`
- [ ] `erna.scripts.process_fact_run_list.py`