[Requirement] Associate events with flipper settings when those are time dependent

In (some, most?) polarized experiments the polarizer and analyzer flipper settings are going to oscillate and measure the sample with different flipper settings. There are a number of things we need to implement to make this work:

• Events need to be associated with the flipper setting the flipper had when the neutron passed through it. If the flipper settings oscillates in sync with the pulses we might be able to make this association per pulse. Otherwise it might be more complicated.
• Events that can't be associated to a flipper setting will have to be masked. (This could happen either if there is some transition time between the two flipper settings, or if there is some uncertainty exactly when the flipper setting changed.)

No files, tests, or entry points are named. Start by tracing how events and time-dependent flipper settings are represented and associated; done means events are assigned to the setting at neutron passage, while events during transitions or with uncertain timing are masked.