Include non-zero transition-time from open<->closed in tof.chopper_cascade

[SimonHeybrock]

This is due to a non-zero guide side. See https://pubs.aip.org/aip/rsi/article/85/11/115103/356359/Choppers-to-optimise-the-repetition-rate (https://backend.orbit.dtu.dk/ws/portalfiles/portal/102784335/1.4900958.pdf) Fig. 3 and, e.g., Eq. (6) and (7).

In practice we might also need an option to define a cutoff, e.g., to ignore the tails below a threshold.