revise data paths / add a data-project config file or class

Currently we use the repo root (HOME) as the root directory for paths to datasets, logs etc.

For the future user, it would be useful to have a global config file or class (one per project) where this root is defined, and from which it is loaded as a constant.

All paths used in the code would be reletive to this root, i.e. instead of HOME/aaad/data/projectname/checkpoints would be ROOT/projectname/checkpoints