Updated examples

[AHsu98]

Updated basic and dataframe splitting examples Fixed some docstrings Changed rate splitting code to have the option to zero out groups with no one in them, this is useful in dataframe splitting, as we may split different parts of the population with different totals, using dummy variables to indicate which group is split with which row Everything should be pretty cleaned up now in preparation to add the draws/pattern uncertainty interface.

[AHsu98]

Force merging here, don't think we need review on this.