Pandas-indexing

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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`

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* Add usage guide to documentation
* Fix :py:func:`~core.semijoin` method

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* Clean up documentation and switch theme (:pull:`1`)
* Add :py:func:`~core.semijoin` method (:pull:`2`)
* Introduce pandas accessor :py:mod:`idx <accessors>` on :py:class:`~pandas.DataFrame`,
:py:class:`~pandas.Series` and :py:class:`~pandas.Index`

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* Initial release