[examples] Add examples with Hermite-type bases

This adds three examples:

* 2d Poisson equation with cubic Hermite-basis and weak enforcement of
  Dirichlet BC (Nitsches method).
* 2d Poisson equation with cubic Hermite-basis and strong enforcement
  of Dirichlet BC utilizing the `FunctionalDescriptor` interface.
* 2d biharmonic equation with nonconforming Morley basis and strong
  enforcement of clamped BC utilizing the `FunctionalDescriptor` interface.