Refactoring `plotSlice` for implementation in other plotting routines

[prisae]

A refactoring of plotSlice, in TensorMesh but also in TreeMesh, is required in order to be used by other plotting routines such as plot_3d_slicer.

plotSlice should have an option to only return the actual plot for that axis, without any decorations such as axis labels etc. This could be a pcolormesh instance or a collection of Polygon-patches or another instance. But just the plot, nothing else.