refactor nodes into standalone file

backend/nodes/plane_level_field.py

@@ -0,0 +1,45 @@
+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="Plane Level")
+class PlaneLevelField:
+    @classmethod
+    def INPUT_TYPES(cls):
+        return {
+            "required": {
+                "field": ("DATA_FIELD",),
+            }
+        }
+
+    RETURN_TYPES = ("DATA_FIELD",)
+    RETURN_NAMES = ("leveled",)
+    FUNCTION = "process"
+
+    DESCRIPTION = (
+        "Fit and subtract a least-squares plane from the data. "
+        "Equivalent to gwy_data_field_fit_plane + gwy_data_field_plane_level."
+    )
+
+    def process(self, field: DataField) -> tuple:
+        data = field.data.copy()
+        yres, xres = data.shape
+
+        x = np.linspace(0.0, 1.0, xres)
+        y = np.linspace(0.0, 1.0, yres)
+        xx, yy = np.meshgrid(x, y)
+
+        A = np.column_stack([
+            np.ones(xres * yres),
+            xx.ravel(),
+            yy.ravel(),
+        ])
+        z = data.ravel()
+
+        coeffs, _, _, _ = np.linalg.lstsq(A, z, rcond=None)
+        pa, pbx, pby = coeffs
+
+        plane = (pa + pbx * xx + pby * yy)
+        return (field.replace(data=data - plane),)