[meshplotlib] refactoring, bugfixing

Besides mostly refactoring the two main things to note here are:

• there is now a function, to plot the outline (of a 1D mesh) on an existing matplotlib Axes -> mpl.plot_contour
• the isometric plot functionality of meshplotlib was dropped, since it was just wrapping the pyvista plots anyway. The 3D example now uses the native pyvista plot in the beginning (if the following is merged to pyvista, we can have interactive plots in the docu: https://github.com/pyvista/pyvista/pull/4938)