Remove dummy nodes.

[Durman]

Addressed problem description

Instead of dummy node a Dependency error is shown.

To add dependency to a node it's enough to add extra class variable.

class SvDelaunay3dMk2Node(SverchCustomTreeNode, bpy.types.Node):
    sv_dependencies = {'scipy'}

Preflight checklist

• Code changes complete.
• Code documentation complete.
• Manual testing done.
• Ready for merge.