Refactor resample step

The aim here is to be able to resample both the fixed and the moving image
of the register step.
Tho this end, the resampling was factorized in a widget so it can be used
twice without any code duplication. The step simply instantiates the
widgets.
The first resampling is done on the fixed image of the registration. The
second resampling is done on the moving image of the registration,
using the same final spacing as given by the first resampling.

Finaly, the dynamic property ignoreWorkflow was added. It allows widget to
stay hidden/visible even though their workflow property would suggest
otherwise. It is useful when ones want to hide part of a widget. Without it
changing the workflow level would most likely set it to visible. With it,
the workflow level is ignored and the widget stays hidden.
Automaticaly hide the background label

When using the PDF segmenter, the output segmentation as (usually) two
labels: the object and the background.
But when using it interactively, the background label can hide all the
image which isn't ideal for user interaction. That's why upo completion of
the PDF segmenter, the background label is set to 0 so only the segmented
mask is left.

At the same time, plug a memory leak on a vtkCollection.
Add Json support for the CLIs

Now, the CLIs first configure themselves with the workflow's input and then
the resource's json files. If an entry already exists in the parameters
dictionnairies its json value is simply ignored. If no corresponding json
file exists, then nothing happens.
Infere the names from json config file

Depending on the task selected, we want to be able to change the names of
the workflow inputs dynamicaly to provide the user with a clearer GUI.
This is done in the intial step. Much like it is done for the json presets,
the naming is stored in a json file named Workflow.json.
The README were consequently updated to reflect this too.
Finaly, the naming was propagated to the LoadDataStep, the ResampleStep
and the SegmentationStep.
Leave ResampleStep part B) enabled

Otherwise it's always disabled until the part A has run. Even in
intermidiate or advanced mode.
Factorize the view settings the advanced settings

They both have been merged together in one widget for clarity. Also,
instead of being under the button, they are now added to the workflow
layout so they appear inside it (above the back/next buttons). The progress
bar was moved as well. This feels and look more natural.
Hide the opacity slider on initial step

The opacity slider is now hidden and disabled on the initial step. It is
shown as soon as the initial step is passed and enabled when the first
step is validated.
ENH: Use volume 2 in pdf segmenter

Add the 2nd resampled volume as an additional input of the PDF segmenter.
Factorize setOutputNode in Resample Widget

This allows a cleaner code and the Resample step to work with isotropic
images as input (the step is automatically validated).
Rename vessel extraction to vessel enhancement

Also revisit this placeholder and update it further now that we know that
it will be used with EnhanceUsingNJetDiscriminantAnalysis.
Add ExtractSkeleton step:

Just a placeholder for now
Add Vessel Extraction Step

Just a placeholder for now.
Refactor settings panel

Set the reload button visible in advanced workflow only.
Also, the volume currently viewed by the user are a little bit more
enforced b re-setting the last viewed volume after each step and everytime
the module is shown.