RGB nifti does not work with `volume-to-precomputed-pyramid` out of the box

[xgui3783]

as of 1.0.0

min repro

volume-to-precomputed-pyramid rgb.nii.gz ./out/

expected behaviour:

converts rgb nifti to 3 channel volume

actual behaviour:

exits confused with why void24 is the data_type of input nifti

comment:

apologies, this one escaped me. I will provide a PR & add tests.