Updating objects and the update stage of AdaptiveSolver

For a variational problem solved using AdaptiveSolver, the members of
the variational problem, the associated boundary conditions, the error
control objects etc need to be somehow updated after each mesh
refinement.

This applies (at least) for

  * FunctionSpace
  * Function
  * BoundaryCondition
  * Form
  * ErrorControl

Taking Form as an example, one could either imagine an in-place update

  Form::update(const Mesh& new_mesh)

or an update returning a new Form on the new_mesh, something like

  const Form Form::update(const Mesh& new_mesh)

or something entirely different.

Once this is in place, the update step of AdaptiveSolver::solve needs
to be implemente