* Add normalized Taylor plot
* Add aggregate_observations argument to taylor plot
* Add new metrics: max_error and willmott's Index of Agreement
* Improve TrackObservation item argument: now takes both int and string
* Improve TrackObservation input check, now require input to be specified if more than 3 items
* Bugfix: normalize NSE metric with obs
* Bugfix: remove class attributes that caused problems with multiple objects
* Add obs_item to fmskill.compare()
* Improved getitem to handle negative indexes and raise correct errors
* Bugfix: fmskill.compare() select item
* Improved metric documentation