Remove defaults from TransformVolumeData.py

[nilsvu]

Instead, prompt the user for missing dataset names. This makes it more predictable and safer which datasets are read and written, and makes it easier to use the tool.

I've been using this tool to compute Christoffel derivs and spatial Ricci tensor in initial data to find horizons:

spectre transform-vol BbhVolume* -k spectre.NumericalAlgorithms.LinearOperators.partial_derivative
# then select 'SpatialChristoffelSecondKind' when prompted

spectre transform-vol BbhVolume* -k spectre.PointwiseFunctions.GeneralRelativity.ricci_tensor
# also select the dataset names when prompted

spectre bbh find-horizon BbhVolume* -l 12 -r 2 -C 8 0 0

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