yucongalicechen output dir

[sbillinge]

closes #22

• initial commit
• changed output_dir to absolute path, changed default to cwd, updated args.output_directory as output directory we defined
• edited help message
• Wrap up set_output_directory as a function and include tests functions
• included docstring and more tests
• added more tests for existing dir and existing file
• added more tests for existing dir and existing file
• deleted the unnecessary test case
• changed to existing_file.touch()
• tests for error messages
• adding richer tests of exception raising
• tweaking help messages in parser for wavelengths
• tweaking tests