merge: Combine datasets

[m-mohr]

Fiboa ‘merge’ command (I think I still don’t have a ticket for this). Should have a command that makes it easy for people to combine data. There’s some of this in support of FTW data release, and that’s likely the ‘easy’ case, where none of the boundaries overlap. But it’d be good to have a way to merge multiple years, like from one of the government data sets, where there’s a ‘latest’ that gets used, and then also the history (this obviously needs a good bit of work to determine the right fiboa extensions, but we could start with trying to merge existing data). And then also to do a merge of two datasets that have overlaps, and have one or two strategies for how to pick which one if there are conflicts. Could do some of those as ‘operations’ - merge is simple, and then people can use operations to clean it up.