unbound local variable if spelling error in config

[Swarchal]

used - plate: instead of - plates:

** parsing config file 'BH_test_config.yaml'
Traceback (most recent call last):
  File "/exports/applications/apps/SL7/python/2.7.10/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/exports/applications/apps/SL7/python/2.7.10/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/gpfs/igmmfs01/eddie/Drug-Discovery/tools/cptools2/cptools2/__main__.py", line 48, in <module>
    main()
  File "/gpfs/igmmfs01/eddie/Drug-Discovery/tools/cptools2/cptools2/__main__.py", line 26, in main
    add_plate_args = parse_yaml.add_plate(yaml_dict)
  File "build/bdist.linux-x86_64/egg/cptools2/parse_yaml.py", line 65, in add_plate
    return {"exp_dir" : experiment, "plates" : plates}
UnboundLocalError: local variable 'plates' referenced before assignment