dataset-specific configuration file

[michaelmack]

Need to rewrite scripts to reference a configuration file that includes dataset-specific paths. This will require updating all existing script to use the configuration files.

Not sure how to best do this. Options:

• Explicit argument to all scripts (e.g., > maget_run_subjects.sh config_hcp.txt).
• A source activate scheme that simply sets dataset paths in environment variables which are then referenced in the scripts. Would need to include checks on paths in scripts