PI_keigen+SMM

[wdhawkins]

• Does not work correctly for anything other than P0. We should throw an exception for anything other than P0. Addressed by Add experimental warning to SMM. Throw an exception if SMM is used with scattering order > 0. #717.
• Should be marked as experimental. Addressed by Add experimental warning to SMM. Throw an exception if SMM is used with scattering order > 0. #717.
• Should be extended to support multiple groupsets.