This implements the segmentation watershed algorithm to InVesalius.

Watershed is a very useful algorithm to segment brain, for example. The
implementation used is from scikit image. Before the segmentation, a
ww&wl is applied and a morphological gradient is applied, to extract the
borders of the image. It can be used to 2D slices or to all slices (via
expand button). It was created a new slice interaction style: WaterShedInteractorStyle.

Starting to using watershed

Added suport to erase a mark

Applying threshold before starting watershed mode

Watershed 3D (only markers from axial)

Watershed 3D

Marking the mask as edited when using watershed tool

Fixed the problem of not deleting watershed interactor style

Not flushing buffer (it's not necessary)

Not hidding wx's cursor when marking to watershed

Removed the vtk colour part from watershed style

Removed the vtk colour part from watershed style

micro-optimization

Removing aux matrix when opening a new dicom dir

not showing watershed markers when mask is hidden

Only doing watershed when user marks back and foreground.

Reloading all mask slice when watershed is expanded

showing (hidding) watershed marks when enabled (disabled)

cleaning undo redo when watershed is applied (TODO create undo redo to watershed)

Interface to select watershed operation
Merge pull request #20 from tfmoraes/watershed_merge

This implements the segmentation watershed algorithm to InVesalius.