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Added
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* Properties `data`, `xreal`, `yreal`, ... to `GwyDataField`.
* `GwyContainer` subclass.
* Docstrings for `gwyfile.objects`.
* `gwyfile.load` convenience method for loading files.
Changed
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* Removed imports of classes from `gwyfile.objects` to `gwyfile`.
* Removed `GwyDataField.get_data()`. Dropped in favor of properties.
* `GwyObject.fromfile` and `GwyObject.tofile` now also take file-like objects.