Fixed whitespace error causing failures in DFSL_parser & Added ability to import custom PyTorch models

[OwenHoffend]

Previous version of DFSL_parser did not consider mapping files containing extra whitespace characters \n and \r, added by some IDEs/OSes. This caused a failing error where the parser would fail to match tokens to anything in the list of string constants. Fixed by adding \n and \r to the list of separation characters. Also added some more descriptive debug printouts to the ParseError calls.

[OwenHoffend]

Added the ability to import custom PyTorch Models into the frameworks_to_modelfile.py script. This behaves in a similar manner to the custom Keras model import functionality which already existed.

[KelvinYang0320]

@OwenHoffend Thank you. That's helpful!