Resolve "Create training & dev sets"

Closes #2 (closed)

• quick introduction into reasoning behind split
• discussed ratio choices
• introduced stratification
• implemented workaround for continuous variables
• investigated equal-width and quantile binning
• implemented Freedman–Diaconis rule and Doane's formula for bin size
• implemented binning adjustment for bridge-classes
• stratification analysis and thoughts on complexity
• adjustments and final splits