st: carryforward updated

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

Hi everyone,
Thanks to Kit Baum, a new version of carryforward has been placed in SSC.

carryforward copies values from one observation to the next, 
replacing missing values with values from earlier observations.
It is a simple operation that you can write easily in one line, but 
this facilitates the process, gives you some additional capability, 
and could make your code more readable.

As I mentioned once some time ago, it is not for imputation, but for 
filling in holes where it is expected that values should persist in 
their sequence, and where the dataset has holes due to the way it was 
constructed (e.g.,by merging). See the help file for a further explanation.

The new feature is that it accepts multiple variables. Also new is a 
"strict" option, affecting how it behaves in regards to excluded observations.

Several users, over the past several years, have asked for the 
multiple-variable capability. I thank them for the suggestion.
--David

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