Last feature index is inconsistent

[mwtoews]

This is an issue with drivers like GPKG which have feature indexes that start with 1 (and not 0). For example poly.gpkg with 10 features:

import fiona

ds = fiona.open("poly.gpkg")

assert ds[0] is None
print(ds[1]["id"])   # 1  <- the first
print(ds[10]["id"])  # 10 <- the last
print(ds[-1]["id"])  # 9  <- whoops!

with a -1 getitem index, it is usually expected to get the last item, but instead the second last item is returned.

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Some ideas for the next development cycle is to deprecate negative indices for getitem, since they are not always zero-indexed, and introduce something like iloc (or irec?) similar to pandas to always retrieve the zero-indexed records.