Structure tensor wrapper

In addition to the wrapper and viz function, I also black formatted some of the my previous merges as I had forgot it. Also increased the width of the local thickness slice index slider to be coherent with the ones in the structure tensor viz function

qim3d/tests/processing/test_structure_tensor.py

+import pytest
+import numpy as np
+import qim3d
+
+def test_wrong_ndim():
+    img_2d = np.random.rand(50, 50)
+    with pytest.raises(ValueError, match = "The input volume must be 3D"):
+        qim3d.processing.structure_tensor(img_2d, 1.5, 1.5)
+
+def test_structure_tensor():
+    volume = np.random.rand(50, 50, 50)
+    val, vec = qim3d.processing.structure_tensor(volume, 1.5, 1.5)
+    assert val.shape == (3, 50, 50, 50)
+    assert vec.shape == (3, 50, 50, 50)
+    assert np.all(val[0] <= val[1])
+    assert np.all(val[1] <= val[2])
+    assert np.all(val[0] <= val[2])
+
+def test_structure_tensor_full():
+    volume = np.random.rand(50, 50, 50)
+    val, vec = qim3d.processing.structure_tensor(volume, 1.5, 1.5, full=True)
+    assert val.shape == (3, 50, 50, 50)
+    assert vec.shape == (3, 3, 50, 50, 50)
+    assert np.all(val[0] <= val[1])
+    assert np.all(val[1] <= val[2])
+    assert np.all(val[0] <= val[2])
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