st: New version of -usesome- on SSC

[daniel klein <klein.daniel.81@gmail.com>]

Thanks as always to Kit Baum, an updated version of -usesome- is now
available from the SSC.

The old version failed to load even a single variable if the list of
variable names in the dataset to be loaded was too long for a local
macro and issued the error message  "too many variables specified"
r(103). This is because the code relied on -describe using- to obtain
a list of variable names.

The updated version fixes this problem and additionally

- supports standard abbreviations for variable names
- supports any type of numlist (bug fix)
- supports -if- (new option)
- returns some 'results'

Best
Daniel
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