Revert "A few updates to stanpredict, addition of experimental WAIC calculation"

This reverts commit fe9d794d6c7a9985a8da2c9b93455852052b1a8b.
Revert "Revert "A few updates to stanpredict, addition of experimental WAIC calculation""

This reverts commit 44dfba24ea7e241bb368bf0f11b4bd4e23e6b84d.