Remove unneeded statistical code

[rlabbe]

See https://github.com/rlabbe/Kalman-and-Bayesian-Filters-in-Python/issues/188, which refers to usage in the book as well as this library.

There are several functions in stats.py that compute things that are implemented in NumPy or SciPy. In some cases my code is faster, but people need to learn the standard libraries, and I shouldn't be maintaining code that duplicates computations that are already implemented and have a far more robust test infrastructure.

[rlabbe]

I've added deprecation warnings.