Plot of quadratic cells

[adtzlr]

Try to implement a fix to hide the internal edges, see pyvista/pyvista#5777.

We need something like

plotter = FieldContainer.plot(hide_internal_edges=True, nonlinear_subdivision=1)

or (more pyvista-like)

plotter = FieldContainer.plot(extract_surface=True, show_edges=True, nonlinear_subdivision=1)

where the edges are shown as feature_edges of a separated mesh if extract_surface=True. Tbh, I don't know what side-effects extract_surface() has.