refactor nodes into standalone file

backend/nodes/fix_zero.py

@@ -0,0 +1,37 @@
+from __future__ import annotations
+import numpy as np
+from backend.node_registry import register_node
+from backend.data_types import DataField
+
+
+@register_node(display_name="Fix Zero")
+class FixZero:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "field": ("DATA_FIELD",),
+                "method": (["min", "mean", "median"],),
+            }
+        }
+
+    RETURN_TYPES = ("DATA_FIELD",)
+    RETURN_NAMES = ("zeroed",)
+    FUNCTION = "process"
+
+    DESCRIPTION = (
+        "Shift data so that the minimum (or mean/median) is zero. "
+        "Equivalent to fix_zero in Gwyddion's level.c."
+    )
+
+    def process(self, field: DataField, method: str) -> tuple:
+        data = field.data.copy()
+        if method == "min":
+            data -= data.min()
+        elif method == "mean":
+            data -= data.mean()
+        elif method == "median":
+            data -= np.median(data)
+        else:
+            raise ValueError(f"Unknown method: {method}")
+        return (field.replace(data=data),)