Check whether raise warning is valid method

[nelse003]

Is this a good way to handle errors? Can it be improved by logging?

get_parameter_from_occupancy_groups(occupancy_groups, parameter_str)

                    raise Warning("Multiple models are present in pdb file. "
                                  "This is not processable with occupancy group selection")
    if parameters == []:
        raise Warning("Parameter may not be recognised,as output list is empty")

[nelse003]

Added logger