Examples and tests for KDE

This PR is to add some tests and examples for the parts of the nonparametric/KDEUnivariate.py, and sandbox/nonparametric/kernels.py

So far I have added a few small tests, checking exception throwing, and testing out the normalization calculation. I've also added an example which shows of the cdf, icdf, survival function and cumulative hazard.

Working on adding some tests for these properties based on values in Stata/R.

Formatting might be off, would appreciate some feedback at some point.