ENH: improved control over IO volume types

It is now possible to add custom volume types to node selectors:
`slicer.modules.SimpleFiltersInstance.scalarVolumeNodeClasses=['...']`

Author: kerim371

SimpleFilters/SimpleFilters.py

@@ -32,6 +32,7 @@ class SimpleFilters:
     import inspect
     __file__ = inspect.getframeinfo(inspect.currentframe())[0]
 
+  scalarVolumeNodeClasses = ["vtkMRMLScalarVolumeNode", "vtkMRMLLabelMapVolumeNode"]
   ICON_DIR = os.path.dirname(os.path.realpath(__file__)) + '/Resources/Icons/'
   JSON_DIR = os.path.dirname(os.path.realpath(__file__)) + '/Resources/json/'
 
@@ -842,7 +843,7 @@ def create(self, json):
 
     self.outputSelector = slicer.qMRMLNodeComboBox()
     self.widgets.append(self.outputSelector)
-    self.outputSelector.nodeTypes = ["vtkMRMLScalarVolumeNode", "vtkMRMLLabelMapVolumeNode"]
+    self.outputSelector.nodeTypes = SimpleFilters.scalarVolumeNodeClasses
     self.outputSelector.selectNodeUponCreation = True
     self.outputSelector.addEnabled = True
     self.outputSelector.removeEnabled = False
@@ -884,7 +885,7 @@ def create(self, json):
   def createInputWidget(self,n, noneEnabled=False):
       inputSelector = slicer.qMRMLNodeComboBox()
       self.widgets.append(inputSelector)
-      inputSelector.nodeTypes = ["vtkMRMLScalarVolumeNode", "vtkMRMLLabelMapVolumeNode"]
+      inputSelector.nodeTypes = SimpleFilters.scalarVolumeNodeClasses
       inputSelector.selectNodeUponCreation = True
       inputSelector.addEnabled = False
       inputSelector.removeEnabled = False