Mixmod weight normalization bug

[aimalz]

When using equal weights of 1 for each component as in the demo, the PPF ignores all components after the primary. We should enforce that weights be fractions summing to one to prevent this. These methods also require unit tests, and the mixmod_examples.ipynb demo should be edited to have an N an N-row ensemble of an M-component mixture model, with N =! M, as users currently can't see the intended arrangement of arguments from the N = M = 3 example.