rotation scan geometry

This is a simple visual explanation of the calculations in move_to_start_w_buffer() and move_to_end_w_buffer() in rotation_scan_plan.py

In order to move at constant speed throughout a scan angle, buffers must be added at the start and end to account for time taken to reach velocity. Additionally, there is a small delay between the fast shutter receiving a trigger and being fully open, when the detector can be triggered.