Remove xarrayutils dependency

[jbusecke]

xarrayutils is currently breaking our tests due to this.

Additionally to fixing the issue over there, I want to get rid of that dependency. I will need to spend some time to redo this with polyfit as indicated here. At which point we could tackle #168 at the same time.

This will take some more time, but should provide great value to the community. If folks here want to push on this at some point, any help would be greatly appreciated.
cc @DamienIrving