st: carryforward updated

[David Kantor <kantor.d@att.net>]

Hi All,
Thanks to Kit Baum, an update to the carryforward module has been 
uploaded to SSC.

The new feature is a -dynamic_condition- option. This allows control 
of the flow of data based on the values being propagated.
Until now, all you had available was the -if- qualifier, which is 
limited to conditions that are knowable at the outset; you could not, 
for example, limit the propagation of values based on a comparison of 
that value to another variable. The new option allows such control.

There are minor changes to the creation of variable labels and notes.

ssc desc carryforward
ssc inst carryforward

Important note: carryforward is for filling in gaps where the value 
are understood to continue from one observation to the next. It is 
not intended for imputing missing values in other situations.

Thanks
--David

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