Cache datasets in XDG directories

[QuLogic]

Problem description

The default directory for caching datasets is currently ~/.xarray_tutorial_data:

xarray/xarray/tutorial.py

Line 15 in 225868d

_default_cache_dir = _os.sep.join(('~', '.xarray_tutorial_data'))

This doesn't follow the XDG standard on Linux, and AFAIK isn't actually invisible on Windows.

Expected Output

It would be better if this followed the XDG standard, i.e., ~/.cache/xarray_tutorial_data (or some other places if you've set some environment variables) and something like %LOCALAPPDATA% on Windows. There are packages like appdirs that handle all this for you.

[shoyer]

Contributions would be very welcome here.

I would be happy to vendor a copy of appdirs to handle this.

[shoyer]

Or even just picking some better, platform specific defaults would be an improvement.

[dcherian]

I think pooch handles this for us now: fatiando/pooch#43

Or if not, it should be fixed there ;)