v0.2.0: Dask and Dataset support

Major updates:

• Lazy evaluation on dask arrays (uses xarray.apply_ufunc and dask.array.map_blocks) 798db9e
• Automatic looping over variables in an xarray Dataset cf8644f.
• Add tutorial notebooks on those new features fed8bef

All user-facing APIs in v0.1.x should still work exactly the same. That said, because some internal codes have changed a lot, there might be unexpected edge cases that break current user code. If that happens, you can revert to the previous version by pip install xesmf==0.1.2 and follow old docs at https://xesmf.readthedocs.io/en/v0.1.2/.