Make xarray a default dependency

[JSKenyon]

Description

I believe that the time may have come to make xarray a core dependency rather than an extra. At present, it is optional as dask-ms implements its own stripped down Dataset. Unfortunately, this is introducing more and more maintenance as these stripped down Dataset objects do not implement a host of xarray.Dataset functionality. This means that writing tests can become frustrating as you have to write the tests anticipating two different Dataset implementations (or skip the tests entirely in the absence of xarray, somewhat defeating the point of writing tests). I am personally of the opinion that the xarray.Dataset representation should be fundamental to dask-ms, even if it is less lightweight.

The purpose of this issue is discussion - does anyone object?