Add tof spotfinding

[toastisme]

Requires #687, #688, #689, #691

• Adds SpotFinder._post_process(), to be called at the end of SpotFinder.find_spots(...), which by default does nothing.
• Adds TOFSpotFinder(SpotFinder), which uses _post_process to add some additional data to the reflection table.
• Adds tof_centroid_px_to_mm_panel to set the third column of xyzObs.mm.value to time of flight