st: Update to -kdens- on SSC

["Jann Ben" <ben.jann@soz.gess.ethz.ch>]

Thanks to Kit Baum, the -kdens- package has been updated 
on SSC.

-kdens- now uses a more accurate approximate variance 
estimator. See

  http://fmwww.bc.edu/RePEc/bocode/k/kdens.pdf

To install the update, type

 . ssc install kdens, replace

ben

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