Energyquantified

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**Bugfixes**

- When invoking ``timeseries.to_dataframe(single_level_header=True)`` the
resulting column index was still a ``MultiIndex`` but with a single level.
Now the resulting column index is a normal ``Index`` type.

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**Dependencies**

- ``requests``: Use the latest v2.x available, as requests is very stable library.
- ``python-dateutil``: Use the latest v2.8.x available.

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**Improvements**

- Add ``eq.metadata.curve()`` which returns a Curve object for the corresponding
curve name.
- Add a section in the metadata documentation on the ``eq.metadata.curve()`` method.
- Add ``eq.metadata.curves()`` and ``eq.metadata.curve()`` to reference page in
the documentation.
- Add ``Area.short_tag`` which is a shorter tag than ``Area.tag``. It is typically
used for TSO areas. Example: The area with tag ``DE-Amprion`` has the short
tag ``Amprion``.
- Changes in capacity allocation for these borders:
- AT-CZ Implicit only
- AT-HU Implicit only
- NO2-GB Implicit
- Add borders:
- RS-XK Explicit
- TR-GE Explicit

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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.

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Implementing Acer's non-standard Gas Day timezone.

**Improvements**

- Define new timezone in ``pytz`` called ``Europe/Gas_Day``. It follows
Acer's Gas Day, which is from 06:00 – 06:00 in CET/CEST. This timezone is
used for the natural gas market in the European Union. Import it with
``from energyquantified.time import GAS_DAY``, or look it up in ``pytz``
like so: ``pytz.timezone("Europe/Gas_Day")``.

**Bugfixes**

- When invoking ``timeseries.to_dataframe(name="foo", single_level_header=True)``,
the resulting column header in pandas' DataFrame no longer includes the
instance identifier. However, the ensemble/scenario name is still appended
at the end.

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**Improvements**

- Increase rate limits.

**Bugfixes**

- Set ``has_instances = True`` in ``CurveType.INSTANCE_PERIOD`` (was ``False``).