Pandas-indexing

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* :func:`~core.formatlevel` and :func:`~core.extractlevel` (or their equivalents
:meth:`~accessors.DataFrameIdxAccessor.format` and
:meth:`~accessors.DataFrameIdxAccessor.extract`) make it easy to combine or split
index levels using format-string like templates; check examples in the guide
(:ref:`Selecting data`) :pull:`13`
* :py:func:`~core.describelevel` superseeds the as-of-now deprecated
:py:func:`~core.summarylevel` :pull:`11`

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* Paper-bag release: Fix new accessors :py:func:`~accessors.IndexIdxAccessor.unique` and
:py:func:`~accessors.IndexIdxAccessor.__repr__` and improve tests to catch trivial
errors like these earlier :pull:`10`

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* :py:func:`~core.uniquelevel` or ``.idx.unique`` returns the unique values of one
or multiple levels. :pull:`8`
* :py:func:`~core.summarylevel` creates a string summarizing the index levels and their
values. Can also be accessed as ``df.idx`` or ``index.idx`` :pull:`9`

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* :py:func:`~core.assignlevel` takes labels from an optional positional dataframe :pull:`5`
* Add :py:func:`~core.dropnalevel` to remove missing index entries :pull:`4`, :pull:`6`

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* Restore compatibility with python 3.8
* Improve typing and add tests for :py:func:`~selectors.isin` and
:py:func:`~selectors.ismatch`

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* :py:func:`~selectors.isin` and :py:func:`~selectors.ismatch` are now callable objects,
which can be composed with the standard ``~``, ``&`` and ``|`` operators to more
complex queries
* :py:func:`~arithmetics.add`, :py:func:`~arithmetics.subtract`,
:py:func:`~arithmetics.multiply` and :py:func:`~arithmetics.divide` in the new
:py:mod:`arithmetics` module extend the standard pandas operations with ``join`` and
other arguments known from :py:meth:`pandas.DataFrame.align`.
They are also available from the :py:mod:`idx accessor <accessors>`.
* Both additions were introduced in :pull:`3`