Robust statistics to stats package

I have added several robust statistics to func.py in the stats package.   These include the median_absolute_deviation, biweight_locater, and biweight_scale.   I've also added tests for each function into the testing package.   

This isn't a future functionality so it doesn't need to be immediately looked at if there are more pressing issues, but I'm also sure there are a few things that need to be tweaked to have everything in the right format so any feedback would be great.