st: Update to -jmp- on SSC

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

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

-jmp- now supports the use of avarage coefficients as
the reference coefficients (as in the original paper 
by Juhn et al.). See the -reference- option in 
-help jmp-.

To install the update, type

 . ssc install jmp, replace

ben

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