Clean up after execution to prevent memory leaks
Touchups for Create mesh step

Add the input combo box as a labelmap only combobox.
Prevent the MergeLabel module and the CreateMesh module to select their
output upon creation when they doso. This the next module that would react
to the node changed event to create an output with a name based on the
combo box's naming scheme and not the input.
Add bone volume material extractor to workflow

This steps takes care of extracting the tetrahedral mesh of the bones from
the entire tetrahedral mesh.
Add Skin volume material extractor step

This step is much similar to the extract bone step. The skin surface is
computed from the volume material extractor on the tetrahedral mesh
obtained on the merged labelmap.
Set the bone and skin models' color based on the labelmap

After the bone and the skin model are generated, automatically attribute
them the same color as the merged labelmap they originated from has.
For the skin, also make it transparent so we can see the bone.
Finally, hide the tetrahedral mesh input so it won't oscure the view..
Enable Toggle Skin Visibility button in armature step