Add new ordschur function. * libinterp/corefcn/ordschur.cc: New file. * libinterp/corefcn/module.mk: Include it in the list of source files. * scripts/help/__unimplemented__.m: Remove ordschur from the list of unimplemented functions. * doc/interpreter/linalg.txi: Add it to the interpreter manual. * NEWS: Mention it. * libinterp/corefcn/schur.cc (schur): Reference it from the documentation of the schur function. Thanks to Carnë Draug for improving the original patch, and to Mike Miller for reviewing it and suggesting improvements.

--- a/scripts/help/__unimplemented__.m
+++ b/scripts/help/__unimplemented__.m
@@ -741,7 +741,6 @@
   "openvar",
   "ordeig",
   "ordqz",
-  "ordschur",
   "outerjoin",
   "padecoef",
   "parseSoapResponse",