limit plots, difference plots, clean up

Purpose

Introducing two new plot types: limit plots (max, min) and difference plots. Also reworked the logic for aspect ratios of plots with meshplotlib and added an example for illustration. Also includes several smaller cleanup changes. Commit-wise review highly recommended

Example

If sensible, please add a screenshot something similar to illustrate the merge request.

docs/examples/howto_meshlib/plot_meshlib_pyvista_input.py

@@ -8,15 +8,10 @@ For this example we create meshes from pyvista surfaces.
 """
 
 # %%
-import numpy as np  # For visualization only
-
-import ogstools.meshplotlib as mpl  # For visualization only
 from ogstools.meshlib.boundary import Layer
 from ogstools.meshlib.boundary_set import LayerSet
 from ogstools.meshlib.boundary_subset import Gaussian2D, Surface
-from ogstools.meshlib.region import (
-    to_region_tetraeder,
-)
+from ogstools.meshlib.region import to_region_tetraeder
 
 # See other examples for different meshing algorithms
 
@@ -30,13 +25,9 @@ surface3 = Surface(Gaussian2D(**args, height_offset=-200), material_id=2)
 
 ls = LayerSet([Layer(surface1, surface2), Layer(surface2, surface3)])
 mesh = to_region_tetraeder(ls, 40).mesh
+# interprets values as categories
+mesh["regions"] = [str(m) for m in mesh["MaterialIDs"]]
 
 # %%
 # Visualize the prism mesh
-
-slices = np.reshape(mesh.slice_along_axis(n=4, axis="y"), (1, -1))
-mpl.setup.aspect_limits = [0.2, 5.0]
-fig = mpl.plot(slices, "MaterialIDs")
-for ax, slice in zip(fig.axes, np.ravel(slices)):
-    ax.set_title(f"z = {slice.center[2]:.1f} {mpl.setup.length.data_unit}")
-_ = fig
+mesh.plot(scalars="regions", show_edges=True)