support alternative execution backends #100
Description
As an early groundhog user, I likely do not have a ready-to-use globus compute MEP available on my HPC machine of choice, but I'd still like to take advantage of the groundhog function interface (either .remote() or .local()).
It should be possible to specify an alternative executor type for either remote or local calls, but it's fine for the globus compute executor to remain the default.
It should be possible to either BYO executor (e.g. hayden's burrow SshExecutor proof of concept) or choose from other "first class" executor types, e.g. (parsl executors).