#272 Improve the interface of "sort"

[Erik Hänel]

Improve the interface of the sort command so that it matches to the common way of calling a command:

sort DATA() -set cols=1[2:3]

This will also require to add the possibilty to restrict the lines to be sorted. Also it would be nice, if one would have the possibility to obtain a sorting index vector with -set index instead of directly sorting the data.

Another requirement is to allow to use multiple columns as hierarchical sorting index columns.

Analysis:
This can partly be achieved by adding another interface to the sort routine. To add the possibility to use more than one column as sorting index one has to include a recursion after the first sorting run, which will go for all elements, which are equal in the first column.

Implementation:
The change was implemented as proposed by the analysis. Implementation tests were passed successfully.

Documentation:
The changes were added to the language files. A new documentation article for the command sort was added.

Tests:
The change was added to the automatic tests. No deviations detected. Change request implemented successfully.