st: Update to -moremata- available from SSC

[Ben Jann <ben.jann@gmail.com>]

Thanks to Kit Baum an update to the -moremata- package is available
from SSC. -moremata- provides various Mata functions. To install the
update, type

 . ssc install moremata, replace

or use the -adoupdate- command.

Changes:

o A new -mm_collapse()- function has been added. The function can be
used to compute summary statistics by subgroups (similar to Stata's
-collapse- command). See

     . help mata mm_collapse()

  for details after updating.

ben
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