prune old history

[knaaptime]

the size of the repository including its history is ~460mb, which is massive considering the file size of the repo (without history) is like 20x smaller. The big inflation is because, in the earliest versions of the package, we tried to store large datasets in the repo. We don't need that 400mb sitting around in the history so I'd like to pick a date and delete the old git history beyond that

I suggest 4/29/2019, which is the date of the first geosnap release. Anything in the history prior to that date, I don't think we need to keep for any reason. If we agree on that date, I think it should be straightforward to follow a process like this one.

Soliciting buy-in @sjsrey

[knaaptime]

cc @ljwolf