feather.m: Overhaul function for Matlab visual compatibility. * scripts/plot/feather.m: Reduce length of arrowhead size to 0.20 of arrow shaft. Change arrow head angle with shaft from 45 to 30 degrees. Use variable 'h' instead of 'retval' to match documentation. Redo input validation. Add %!error input validation tests.

#include <octave/oct.h>

DEFUN_DLD (helloworld, args, nargout,
  "Hello World Help String")
{
  int nargin = args.length ();

  octave_stdout << "Hello World has "
                << nargin << " input arguments and "
                << nargout << " output arguments.\n";

  return octave_value_list ();
}