improve DGS single crystal reduction code

by following examples in https://github.com/mcvine/singlextal-diffraction-kernel-validation/blob/8e80eeaf0c2abd9ec0588617e3869e536523403e/ARCS-Si/sim.ipynb

Andrei improved the Mantid code so that the reduction can be done in arbitrary directions. Should use the new mantid code.