This is an attempt to extend the variable order functionality to 3D, partially addressing issue #6 (closed).
I tested this for all element types using the channel tests of the incompressible solver (each element having a fixed order in all 3 directions). All tests worked, even in parallel. It would be nice if someone who actually uses the 3D solver could test this with a larger case.
Once we have some confidence in this, I will probably extend DriverAdaptive to 3D, since that should be straightforward.