drop() method to drop obs/mod/variable (reverse sel)

pandas and xarray allow users to drop part of the dataset with a drop method - see e.g. https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.drop.html. Which is opposite to `sel()` but should have the same syntax. The drop method should return a new Comparer or ComparerCollection with all data except the dropped.

Two new methods should be added to Comparer and ComparerCollection, respectively, both named drop(). Add them below sel() method https://github.com/DHI/modelskill/blob/db2f7c2d785d36e7bcef0c88c2269cbf10002ac7/modelskill/comparison/_comparison.py#L838 and https://github.com/DHI/modelskill/blob/db2f7c2d785d36e7bcef0c88c2269cbf10002ac7/modelskill/comparison/_collection.py#L301