Conversion to xarray

[wpreimes]

Add a function to each component (sensor, station, network, network collection) to transform it into an xarray object.
This would not only allow using all the features that come with xarray, but also to e.g. store the ismn data in netcdf etc format.

[wpreimes]

Done in #87 . A similar function for NetworkCollection or ISMN_Interface(..., as_xarray=True) could still be added.

[wpreimes]

Add CLI to transfrom all networks into netcdf format