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.

#include "mex.h"

void
mexFunction (int nlhs, mxArray *plhs[],
             int nrhs, const mxArray *prhs[])
{
  mexPrintf ("Hello, World!\n");

  mexPrintf ("I have %d inputs and %d outputs\n", nrhs, nlhs);
}