RF+BF: always use absolute path for data file in Builder scripts

- needed after removing os.chdir prior to running a script from builder.py / experiment.py
  IMHO always working with abs paths is an improvement
- allow user to specify a relative or absolute path is settings:
  use the users's explicit abs path if its abosolute, otherwise use the abspath of the script's dir
- BF: handle case of user deletes data file name