Major Changes
* Added support for shape-0 datasets.
* Fix a memory leak where in-memory datasets would not be garbage collected.
* Added support for empty datasets (size 0).
* Make sure versioned data is read-only after closing and reopening the file.
* Allow deleting groups and datasets between versions. Note that currently
dataset metadata cannot change between versions, even if they are deleted in
between.
Minor Changes
* Most tests now use a temporary directory instead of writing a file in the
current directory.
* Fix logic for handling trailing slashes with `in`.
* Automatically create intermediate groups when creating a dataset.
* Make indices that should give a scalar object do so instead of giving a
shape () array.