is_unit(q) yields True if q is a pint unit or a string that names a pint unit and False
      otherwise.

def is_unit(q):
    '''
    is_unit(q) yields True if q is a pint unit or a string that names a pint unit and False
      otherwise.
    '''
    if isinstance(q, six.string_types):
        try: return hasattr(units, q)
        except: return False
    else:
        cls = type(q)
        return cls.__module__.startswith('pint.') and cls.__name__ == 'Unit'