Convenience function wrapper for convert. Takes a single ID and converts it from from_type to to_type.
       The return value is the ID in the scheme of to_type.

def convert_single(ID, from_type, to_type):
    '''Convenience function wrapper for convert. Takes a single ID and converts it from from_type to to_type.
       The return value is the ID in the scheme of to_type.'''

    if from_type not in converter_types:
        raise PubMedConverterTypeException(from_type)
    if to_type not in converter_types:
        raise PubMedConverterTypeException(to_type)

    results = convert([ID], from_type)
    if ID in results:
        return results[ID].get(to_type)
    else:
        return results[ID.upper()].get(to_type)