st: ipshin bugfix

The -ipshin- (Im, Pesaran, Shin panel unit test routine) of Baum and Bornhorst has been corrected. When an explicit list of lag lengths was given, the resulting macro was malformed. That has been corrected in the version available from the SSC archive.

Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html

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