Refactor GaNDLF Task Runner to pass in CSV files in DataLoader

[Patrick Foley]

The GaNDLF Task Runner currently expects the training and validation CSV files to be passed through the TaskRunner, which is staticly set in the plan. This has the downside of requiring that each collaborator has the same file name. Instead, these CSV files should be passed in through data.yaml, where they are configurable for each collaborator.