Draft: cast_volume: add an option to apply percentiles only on positive values.

The initial goal of this PR was to ignore (on user request) negative values when computing percentiles during volume-cast.

But it turned out there was some scores here and there that should be fixed on this PR.

It also intend to generalize PartialHistogram for any type of volume (defined in tomoscan at least). this is why the signature of PartialHistogram has been changed to work on a volume instead of an HDF5 url.

At the end it is sadly way more complex than expected. @paleo if you want we can have a look together might be simpler.

TODO

• add option to compute percentiles on positive value only
• rework VolumeHistogram to generalize it for any tomoscan volume
• add test
• add documentation

close #446

Signed-off-by: payno henri.payno@esrf.fr