Jul xarray unstructured

[Boorhin]

I have tried to adapt the netcdf4 unstructured reader to the xarray method. In addition xarray will be able to read CF compliant files with other engines such as "scipy", "pydap", "h5netcdf", "pynio", "cfgrib", "pseudonetcdf" and "zarr" according to documentation: http://xarray.pydata.org/en/latest/generated/xarray.open_dataset.html#xarray.open_dataset

The modifications are minor. The only issues with xarray are linked with dimensions and variables having the same names. This is a known bug and I believe the user could always fall back onto the regular netcdf4 reader.