st: mvsumm, mvcorr updated

[Kit Baum <baum@bc.edu>]

Ernest Berkhout helpfully identfied a bug in Baum and Cox's routine 
mvsumm (moving summary) when -if- or -in- was specified. That bug has 
now been flattened, along with a parallel bug in the derivative work 
-mvcorr-. Both mvsumm and mvcorr may be updated from ssc.

Kit

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