Align Paraview multiblock filename with other volume output file names.

[danielmayer]

Proposed Changes & related work

See issue #1928:

The filename of the Paraview multiblock is set to the case name / config filename. For all other volume outputs, the filename is specified with 'VOLUME_FILENAME' in the config file. See e.g. the Paraview XML filename. This is counter-intuitive and the user is not able to change the multiblock filename in the config file.

This PR aligns the filename handling of the Paraview multiblock output with the other volume outputs, besides calling the config->GetUnsteady_FileName function instead of the config->GetFilename function which is used for the other volume outputs.

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