usort 1.1.3 for SSC

This program is a byable sort command, which allows a) custom first and last substrings, including system (.) and all remaining missing values, b) gsort-like syntax for the ascending and descending order, as well as c) conditional [if] and range [in] sorting. The program is built around the Stata sort command and adds the data-sorted flag (sorted by) to the dataset if all rows are selected and applies Mata _collate() otherwise. Sorting large datasets might be taxing on machine memory or disk space.