Add different grid download formats

[rabutler-usbr]

Consider adding ability to download data in formats other than .bil:

https://prism.oregonstate.edu/documents/PRISM_downloads_web_service.pdf

[rabutler-usbr]

grib2 no longer available; the API docs will be updated soon.

Need to think through how adding ability to download different formats will affect the entire package as all functions expect file names to be formatted a specific way. Additionally, the plotting and other functions expect reading in .bil, so have to think through that too.