Remove obsolete call to rpart::rpconvert.

This was a compatibility function to convert objects from older versions of rpart.
It was droppd in rpart itself in version 4.1-0 (Nov 2012).
Remove method for "date"-class in package survival.

class date was removed from package survival long time ago (at least before ver. 2.35-2, Mar 2009), presumably
in favor of the "Date" class in package:base.
Add myself as contributor