Add setpoint scheduling

[rosecodym]

This is a re-implementation of #46. It needs a backwards compatibility story.

[rosecodym]

@samuelduchesne feel free to take this over and implement whatever backwards compatibility logic you want.

[samuelduchesne]

Hi @rosecodym! I was taking a look a this. What do you mean by backwards compatibility logic?

[rosecodym]

I just want to make sure that if someone loads an old library file and attempts to access these properties, nothing stupid or dangerous will happen. This version might do that, but I didn't get around to testing it.

[samuelduchesne]

I see! I'll make a few tests and report back. One thing I did encounter is that for fields with a drop-down (schedules for example), UMI is not happy when the library is loaded if the drop-down is empty. Is there a way of allowing empty fields?

[rosecodym]

Sorry, I don't follow. UMI doesn't load drop-downs; it loads serialized library JSON files. What's in the file that's causing problems?