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More gas data preparations.
**Improvements**
- Add ``Place.areas`` (list of areas), as some places (i.e. gas interconnectors)
are places on borders and should be listed for both areas.
- Add three new place types: ``PlaceType.GAS_STORAGE``,
``PlaceType.GAS_LNG_TERMINAL`` and ``PlaceType.GAS_INTERCONNECTOR``
- Add ``Curve.commodity`` (str) which is either ``Power``, ``Gas``, ``Coal``,
``Oil``, ``Carbon`` or ``None`` at this time.
- Add a ``commodity`` filter for ``eq.metadata.curves()``.
**Deprecations**
- Add ``Place.area`` is deprecated and will eventually be replaced by
``Place.areas``. It will be removed in a future release.