dump_demos.m: Check if file exists before demo and add timing. * dump_demos.m: Skip demo if output print file already exists. Add tic/toc timing for demo (calculation + plotting) and printing. This allows benchmarks and finding regressions which causes slow plotting or printing.

%!function r = f (x)
%!  if (ischar (x))
%!    r = x;
%!  else
%!    error ("expecting character string");
%!  endif
%!endfunction

%!assert (eval ("f 10;"), "10");