Add __len__ method to Index

[adamjstewart]

This PR adds a __len__ method to Index, defining the length of an index as the number of entries in that index. This is analogous to the definition of length for Python sets.

This PR also documents and tests the get_size method which provides identical functionality. This method has never been documented before. Should we deprecate or remove it in favor of __len__? It's definitely more Pythonic to rely on these kinds of magic methods instead of get_* and set_* methods.

Closes #194

@hobu

[hobu]

Should we deprecate or remove it in favor of __len__?

Deprecate is fine. Rtree comes from a time before there were properties in Python 😄, which is why there's get_*-style stuff.

It's definitely more Pythonic

What was Pythonic 15 years ago is not necessarily the same thing as today. Rtree could definitely use a thorough once over in that regard.

[EwoutH]

For some reason this warning is still triggered in rtree's own CI.

=============================== warnings summary ===============================
rtree/index.py::rtree.index.Index.__init__
  /home/runner/work/rtree/rtree/rtree/index.py:284: DeprecationWarning: index.get_size() is deprecated, use len(index) instead
    warnings.warn(

Is this expected? Could it be that a test is out of date?

[adamjstewart]

Fixed by #323