Structure tensor visualizations

Update of the structure tensor visualizations available through qim3d.processing.structure_tensor(vol, visualize = True).

• Added new third subplot with coloring according to orientations (saturation given by z-component)
• Fixed coloring of original two subplots, such that they correspond to the new coloring
• Updated documentation for qim3d.processing.structure_tensor and qim3d.viz.vectors (and replaced old gif)
• Updated Structure Tensor example notebook in docs/notebooks/structure_tensor.ipynb