volume.from_pointset needs revision

[dickscheid]

In the current implementation c72ece9, the siibra.volumes.from_pointset method seems to have two issues:

• It sets a label value to the pixels before gaussian smoothing, but the intended operation is a KDE which assumed to smooth a peak with value 1 (Smoothing of labels is not a proper operation anyways). So independent of the selected label, the smoothing should be based on setting pixels to 1.
• The function states that the kernel bandwidth is in millimeter, but applies it to voxels irrespective of the physical resolution. A translation form millimeter to voxels should be added so the bandwidth unit applies also for image resolutions that differ from 1mm.

[AhmetNSimsek]

#529 fixes the first task