update (adjoint) Navier-Stokes terms for 2D

- update convect_build_vtg, convect_build_vtbg() for 2D
new examples/navier_stokes/navier_stokes2d.py + test
update Probe.probe() to report non-finite probe points
new MultiProblem solver: allows "conjugate" solution of subproblems
new: MultiProblem example
new: mesh files for MultiProblem example