Minor comment improvements. Refs #8537.
Use investigationID NOT datafileName to obtain datasetID. Refs #8537.

- Changed the query so that rather than searching for a dataset based on a datafile name we now use the investigationID (which is provided to the user in the GUI).
- I also updated all related variable names in each catalog for readability.
Simplify and tidied CatalogPublish algorithm. Refs #8537.

- Use investigation number instead of datafile name for searching.
- Removed unused method (extractFileName).