ObservationVolumeWriterParquet is redundant with ParticleWriterParquet

The ObservationVolumeWriterParquet class is essentially a clone of the ParticleWriterParquet. It can be removed and replaced. The constructor ObservationVolume<TTracking, TVolume, TOutput>::ObservationVolume(TVolume vol) can just be given the latter writer and will function the same.

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