build fixes

Author: mlavik1

Assets/Scripts/Gradients/CentralDifferenceGradientComputator.cs

@@ -1,7 +1,4 @@
-using itk.simple;
-using openDicom.Image;
 using System;
-using System.Runtime.InteropServices;
 using UnityEngine;
 
 namespace UnityVolumeRendering

Assets/Scripts/Gradients/GradientComputator.cs

@@ -1,4 +1,6 @@
+#if UVR_USE_SIMPLEITK
 using itk.simple;
+#endif
 using System;
 using System.Runtime.InteropServices;
 using UnityEngine;