Audinterface

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* Changed: rename
``audinterface.Feature.process_unified_format_index``,
``audinterface.Process.process_unified_format_index``,
``audinterface.ProcessWithContext.process_unified_format_index``
to
``audinterface.Feature.process_index``,
``audinterface.Process.process_index``,
``audinterface.ProcessWithContext.process_index``

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* Fixed: ``audinterface.Feature`` allow
``win_dur=None`` with ``unit="samples"``

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* Changed: store ``Process.channels`` always as a list
* Changed: keep ``Feature.win_dur`` and ``Feature.hop_dur`` in original format

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* Added: arguments ``channels`` and ``mixdown`` to
``audinterface.Process``,
``audinterface.ProcessWithContext``,
``audinterface.Feature``,
``audinterface.Segment``
* Removed: ``channel`` argument from all ``process_*`` functions

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* Fixed: ``audinterface.Feature.__call__``
always returns ``numpy.ndarray``

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* Changed: ``audinterface.Process.process_unified_format_index`` and
``audinterface.Feature.process_unified_format_index``
support filewise index