read_nexrad_keys has permission issues due to tempfile

[rcjackson]

On a Windows machine, the tempfile that is used by read_nexrad_keys is not readable by boto.s3 because it does not have permission to access it. This would indicate to me that it tries to overwrite the temp file that is locked. I think a small modification to manually create and delete the temp file would probably be a workaround to make it compatible with boto.s3.

[mhpicel]

Thanks for opening the first TINT issue! I will look into this and keep you updated. Feel free to submit a PR with your own fix if you feel inclined.