Use xarray as data structure

[thangleiter]

xarray provides a labelled multidimensional array that adds labels to numpy arrays' axes, very similar to pandas dataframes with than two dimensions.

Implementing this as the standard data structure would probably help to make adoption of this package, and writing code that interfaces with it, easier since the many dimensions of e.g. a control matrix or a filter functions that correspond to different physical entities would be labelled in a human-readable way.

The possiblity of incorporation probably stands and falls with if xarray provides a einsum implementation since we strongly rely on optimized paths when contracting multidimensional arrays.