Returns a "histo" class object. This is a list including:
$patient: set to vol$patient.
$patient.name: set to vol$patient.name.
$patient.bd: set to vol$patient.bd.
$patient.sex: set to vol$patient.sex.
$file.basename: set to "".
$file.dirname: set to "".
$object.name: set to "".
$object.alias: alias of the histo object.
$frame.of.reference: set to vol$frame.of.reference.
$ref.pseudo : set to vol$ref.pseudo.
$modality : set to "histo".
$description: description of the histo object.
$creation.date: set to Sys.Date.
$nb.MC: set to 0.
$breaks: vector breakpoints
$mids: vector of cell centers.
$mids.unit: Character string, representing the unit of the abcissa
of the histogram. For instance, "Gy", when vol is a rtdose.
counts: count of voxels whose value is included in the limits
defined by $breaks.
dV_dx: differential histogram, expressed in cm^3ascii by voxel units,
at each $mids.