resource download fails in develop install

[kajiglet]

If you 'python setup.py develop' install harmonica into your environment, then an egg-link in site-packages is added to point to the code and therefore, the data directory for downloaded resources also.

After downloading a new resource, the file or extracted files are not written into the data directory. I surmise that the egg-link is breaking this. harmonica will fail when xarray attempts to read the non-existent data files.