Qa4sm-preprocessing

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* Smos package fixes `29 <https://github.com/awst-austria/qa4sm-preprocessing/pull/29>`_ `#30 <https://github.com/awst-austria/qa4sm-preprocessing/pull/30>`_ `#31 <https://github.com/awst-austria/qa4sm-preprocessing/pull/31>`_

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- FRM4SM Release 2 version

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- new subpackage ismn_frm
- level 2 and timeseries data readers
- bug fixes

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- renaming of keyword ``latdim`` to ``ydim`` and ``londim`` to ``xdim`` in the
reading package
- detection of coordinates based on CF conventions
- better handling of time offset variables

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- more features for ``DirectoryImageReader``
- handling of "image" files with multiple time steps
- better documentation for subclassing
- renaming of other readers
- ``XarrayImageStackReader`` to ``StackImageReader``
- ``XarrayTSReader`` to ``StackTs``
- ``repurpose`` function for image readers
- improved test coverage

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- handling empty data frame added (PR `7 <https://github.com/awst-austria/qa4sm-preprocessing/pull/7>`_)
- Discard attrs (PR `6 <https://github.com/awst-austria/qa4sm-preprocessing/pull/6>`_)
- Sub-daily to daily averaging methods (PR `5 <https://github.com/awst-austria/qa4sm-preprocessing/pull/5>`_)
- unified the reader backend; everything is using read_block now (PR `4 <https://github.com/awst-austria/qa4sm-preprocessing/pull/4>`_)
- a few fixes for unstructured and curvilinear grids (PR `3 <https://github.com/awst-austria/qa4sm-preprocessing/pull/3>`_)