Statsmodels package is only used for one function

Hey,

as mentioned in #453 there is a possibility of reimplementing the ECDF estimator from statsmodels in a small function that also runs faster (50-60%) as we are doing the estimation in place and do not use it further.

As you can see from this search, the statsmodels package is only ever used for its ecdf estimator.

Once the tests are through, I will open a pull request for the updated version which does not require statsmodels any more.

Thanks in advance Lucas