3d removal of background

https://www.notion.so/qim-dtu/3D-remove-background-function-fac8e64da886429eacd9520bc7be0610?pvs=4

implemented into existing filters framework

qim3d/processing/__init__.py

@@ -2,4 +2,5 @@ from .local_thickness_ import local_thickness
 from .structure_tensor_ import structure_tensor
 from .filters import *
 from .detection import *
+from .operations import *
 from .cc import get_3d_cc