Enable a third type of global mesh derefinement

[solin]

Right now the mesh can be reset to basemesh and polynomial
degrees of all elements returned to P_INIT, or one or more refinement
layers can be shaved off and the polynomial degree of all elements
reset to P_INIT. The latter is more efficient than the former, but
what we really need is to shave off one or more layers of refinements
in such a way that the polynomial degrees return to where they were
before. Then the adaptivity with dynamical meshes will be much faster
than now.