Make it possible to save the .radar.sqlite file into the DAVIT_TMPDIR

Use the DAVIT_TMPDIR environment variable if defined.
If it is not use the defined HOME directory.

When HOME directories are a network mount, its preferable to stash
reusable davitpy files on a local filesystem for faster access.

 Changes to be committed:
	modified:   pydarn/radar/__init__.py
	modified:   pydarn/radar/radInfoIo.py
	modified:   pydarn/radar/radStruct.py