Observation factory function

[jsmariegaard]

A factory function for creating an appropriate observation object based on the data and args.

If 'x' or 'y' is given, a PointObservation is created.
If 'x_item' or 'y_item' is given, a TrackObservation is created.

>>> import modelskill as ms
>>> o_pt = ms.observation(df, item=0, x=366844, y=6154291, name="Klagshamn")
>>> o_tr = ms.observation("lon_after_lat.dfs0", item="wl", x_item=1, y_item=0)

Equivalent to ms.model_result