Update default value of weights kwarg in spatial_avg()

[tomvothecoder]

Is your feature request related to a problem? If yes, Please describe

Related issue: xCDAT/xcdat-validation#23 (comment)

weights defaults to None, which can confuse the user and make it seem like weights aren't automatically generated.
It should be set to weights="generated", which performs the same behavior as if it were None.

Also, does it make sense to handle a np.array of weights in spatial_avg()? Currently, it requires custom weights to be a DataArray with the same spatial axis dimensions and sizes as the data variable DataArray. If we want to accept np.array we'd have to implement a method that creates the weights DataArray from this array so that it can be correctly applied to the data variable (still have to think more about the implementation if it makes sense or is possible).

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[tomvothecoder]

Closed in #154