Have Earth Engine calculate coordinate information.

[copybara-service]

Have Earth Engine calculate coordinate information.

Fixes #57. Per the suggestion of @raspstephan, we avoid computing lat/lng information ourselves and instead pull our coordinate information via RPC calls into EE. There didn't seem to be a good way to do this within get_info(), so this introduces two additional IO calls. This should only add overhead to the open_dataset() call (i.e. once) and not to every data index operation.

Running benchmarks now, so far:

open_dataset():avg=33.11,std=8.23,best=22.81,worst=48.00
open_and_chunk():avg=37.61,std=8.79,best=23.95,worst=53.59

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