fix mutability for parameter defaults + add unit tests

[eonu]

• Fix CategoricalHMM and GaussianMixtureHMM parameter defaults for params/init_params being modified.
• Fix CategoricalHMM and GaussianMixtureHMM unfreeze() calling super().freeze() instead of super().unfreeze().
• Change load_digits numbers parameter name to digits.
• Fix serialization/deserialization for _KNNMixin when weighting=None.
• Change SequentialDataset properties to not return copies of arrays.
• Remove SequentialDataset.__eq__.
• Change HMMClassifier prior default to None.
• Add unit tests.