This commit improves the user interface adding support for choosing
the conversion mode.

Available conversion modes are now:
* vector (RGB, RGBA) images to luminance (default mode keeping backward
  compatibility with original module implementation)
* vector images to single component (user select which component to extract)

To support reusing of components by other scripted modules, a logic
and widget have also been created:
* VectorToScalarVolumeConversionModeWidget
* VectorToScalarVolumeLogic

Co-authored-by: Jean-Christophe Fillion-Robin <[email protected]>
Reviewed-by: Andras Lasso <[email protected]>
Reviewed-by: Csaba Pinter <[email protected]>
Reviewed-by: Steve Pieper <[email protected]>

Add new method to compute the average of all the components of a
VectorVolume. Also add parameterNode awareness, and general reformat that
deserve a new commit.

Co-authored-by: Jean-Christophe Fillion-Robin <[email protected]>