Make dropout a required field in config?

[AUdaltsova]

Following discussion from #45

dropout_timedeltas_minutes is not a required field currently. I think it should be required because imo the whole concept of dropout being "off" is a bit of an illusion: there isn't really such thing as ambiguous delay, you either set it to something or it defaults to instant availability. Which is an "off" in the sense that it's not delayed, but it's still a very specific scenario that you are committing to, so I think it would be nice to think of that and consciously put 0 instead of thinking oh well, just do whatever it is you do

My initial instinct was to put [0] as the "off" version. I recognise it's slightly icky in terms of [ ] or None looking better there, but again, I think 0 should be a conscious choice so it should be put there by the user. But also I recognise that currently None is handled separately so putting [0] will lead to unnecessary computations (albeit probably quite small), and changing any of that might require some reworking of how dropout is done (which is possibly getting revamped anyway?)