Maybe change PadoDataset.__getitem__ exceptions

[ap--]

We provide a Mapping[ImageId, PadoItem] interface with __getitem__ and __len__
in which we correctly (and intuitively) raise KeyError(image_id)

Now additionally we'd like to provide a Sequence[PadoItem] interface, again via __getitem__ and __len__ and again correctly (and to me intuitively) raise IndexError(int_idx)

If the two are merged (as it is now), the user must understand which Exception to catch when using either integer indexing or ImageId indexing (i.e. the Sequence interface or the Mapping interface).

While intuitive to some, this behavior might confuse people who haven't read the collections.abc module a few times. Which is why I believe there's value in separating the two interfaces explicity.

So maybe offer Mapping[ImageId, PadoItem] on the dataset, and Sequence[PadoItem] via a proxy attribute.