Pandas-indexing

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* Add ``mode="append"`` and ``mode="return"`` arguments to
:func:`~core.aggregatelevel`, which extend the dataframe with the aggregated
data or return it :pull:`39`
* Add ``fail_on_reorder`` argument to :func:`~core.semijoin` to raise a
``ValueError`` if the resulting data is not in the order of the provided
index (helpful in conjunction with :func:`~core.assignlevel`) :pull:`37`
* Enhance :func:`~core.concat` to also concatenate :class:`~pandas.Index` and
:class:`~pandas.MultiIndex` objects :pull:`37`

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* Revise :mod:`arithmetics` module:

* Add all standard binary ops: ``add``, ``sub``, ``mul``, ``pow``, ``mod``,
``floordiv``, ``truediv``, ``divmod``, ``radd``, ``rsub``, ``rmul``,
``rpow``, ``rmod``, ``rfloordiv``, ``rtruediv``, ``rdivmod``
* Support in-call assignment of individual levels using ``assign`` argument,
like ``div(generation, capacity, assign=dict(variable="capacity_factor"))``
* Add a unit-aware variant for each binary op, like
:func:`~arithmetics.unitadd`, or :func:`~arithmetics.unitmul`, which
updates homogeneous units automatically with the calculation

* Add ``fill_value`` argument to :func:`~core.semijoin` for filling joining gaps
* Add :func:`~core.aggregatelevel` for aggregating individual level labels; in
:pull:`32`
* Fix :func:`~core.formatlevel` to create a simple single-level index, if only
a single index remains :pull:`29`
* Add :func:`~core.to_tidy` for converting a time-series data-frame to tidy
format, as expected by plotting libraries like seaborn or plotly express; in
:pull:`31`.

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* Rename pandas accessor to ``.pix`` (``.idx`` is as of now deprecated, but
available for the time being) in :pull:`27`.
* Fix :func:`~core.projectlevel` on columns of a ``DataFrame`` :pull:`28`

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* Units can be converted with :func:`~units.convert_unit`, like f.ex.
``convert_unit(df, "km / h")`` or with ``convert_unit(df, {"m / s": "km / h"})``
to convert only data with the ``m / s`` unit
* If the openscm-units registry is registered as pint application registry then
emission conversion between gas species are possible under the correct
contexts:

.. code-block:: python

from pandas_indexing import set_openscm_registry_as_default, convert_unit

ur = set_openscm_registry_as_default()
with ur.context("AR6GWP100"):
df = convert_unit(df, "Mt CO2e/yr") or df = df.idx.convert_unit("Mt CO2e/yr")

* To use unit conversion, you should install with ``pip install "pandas-indexing[units]"``
to pull in the optional ``pint`` and ``openscm-units`` dependencies
* For more information about unit handling, refer to
:py:mod:`~pandas_indexing.units` or check the code added in :pull:`17`
* Documentation fixes: MyST notebook rendering from :pull:`20` and new docs for
:func:`~core.extractlevel` in :pull:`21`.
* Bug fixes: :func:`~core.semijoin`, :func:`~core.concat` and :func:`~selectors.ismatch`
are working again as advertised :pull:`21` and :pull:`24`.

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* Compatibility release to re-include Python 3.8 support and fix CI testing
* :func:`~accessors.IndexIdxAccessor.extract` gains single-level index support
* Minimal doc improvements

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* :func:`~core.extractlevel` can be used on non-multiindex, like
f.ex. ``extractlevel(df, "{sector}|{gas}")`` :pull:`18`
* :func:`~selectors.isin` accepts callable filters :pull:`16`, f.ex.
``df.loc[isin(year=lambda s: s>2000)]``
* New function :func:`~core.concat` makes concatenation level aware :pull:`14`