Initial commit of sample statistics functions to the stat package
Incorporated PR comments
Renamed SortWeightedData to SortWeighted
Flipped the names of Quantile and Percentile. Added CumulantKind to Quantile and Percentile functions.
Moved Quantile panic behavior into the switch case
Changed formattign on histogram function. Changed name of Percentile to CDF and improved panic message

Lists should be separated out into their own block (see the reflect package).
It was decided in code review that Quantile and Percentile are confusing
so it was decided to change it to Quantile and CDF
Proper indentation of CumulantKind behaviors for CDF and Quantile
Merge pull request #1 from gonum/featureinitialcommit

Initial commit of sample statistics functions to the stat package