Pandas-indexing

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* **BREAKING** :mod:`accessors` is imported implicitly. User code does not need to
import it any longer :pull:`45`
* Add continuous testing for python 3.12 :pull:`47`
* Add :attr:`~selectors.All` and :attr:`~selectors.None_` completing selector group
:pull:`46`
* Fix type hints of function :func:`~core.aggregatelevel` :pull:`44`
* Switch from black to ruff for formatting and update pre-commit versions :pull:`43`

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* The new :func:`~core.assignlevel` argument ``ignore_index=True`` prevents the
dataframe and series alignment which became the default in v0.3 (yesterday),
since there are valid use cases of the old behaviour :pull:`41`

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* **BREAKING** :func:`~core.assignlevel` aligns :class:`~pandas.Series` and
:class:`~pandas.DataFrame` before adding them to the index :pull:`38`
* Address pandas 2.1's ``FutureWarning``s and improve test coverage :pull:`40`

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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`