ENH: Added icon loading for scripted module base class
Python scripted modules that use the ScriptedLoadableModule base class now load their icon from Resources/Icons/.png. If the icon file does not exist then the default icon is kept. Modules can override the icon setting in the module init function to use any other custom icon.
Updated extension wizard template and extension test.
Updated one more module (VectorToScalarVolume) to use ScriptedLoadableModule base class.