Bug: Error in `run_prism` if args.output already exists

[omgol411]

In main.py, in the functionrun_prism, if the args.output already exists, then the first condition is unmet and output_dir is still None. os.makedirs(output_dir) will throw the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen os>", line 210, in makedirs
  File "<frozen posixpath>", line 103, in split
TypeError: expected str, bytes or os.PathLike object, not NoneType

[omgol411]

This argument is needed in ihm_parser.py.

[omgol411]

Fixed in #45