improve chunk handling

[chrishavlin]

yt_xarray currently loads in data on a single yt grid. For uniform grids, it should be straightforward to chunk it as well as align the yt grids with any existing xarray chunks (e.g., when xarray is using a dask backend) (NOTE: This is done). Stretched grids however, might need some upstream changes in yt.

[chrishavlin]

#30 addresses part of this: chunking for uniform grids. It works with dask arrays!

To close this issue for the v0.2.0 release, just need to add the option to auto-align the dask and yt chunks.

Future releases could add the functionality for stretched grids, but some work upstream in yt may be necessary for that.