initial test code for tapered guide

[yxqd]

In preparation for implementing the guide_tapering component in numba. For reference, a mcstas implementation is here: https://github.com/mcvine/mcvine/blob/c06eda8c47d4fa69c1c600c18d17a7aa2467a7e7/packages/legacycomponents/mcstas2/share/McStas-Components/optics/Guide_tapering.comp

Plots at the exit of the guide:

• Image
• Acceptance diagram