Add a Neumann partitioned-heat solver

[davidscn]

..not sure if we really want to add it here. I mainly created it in order to investigate precice/tutorials#214.

For simplicity, the solver is currently compiled into the elasticity executable and uses the same parameter file. Some clean-ups are still open.

Open TODO (as a reminder):

• [ ] take time dependency of the coupling data into account and clean-up data reloading

[uekerman]

Adding deal.II to the partitioned-heat-equation tutorial as well has some added value in terms of validation and system tests. Depends on what's the necessary effort and what are drawbacks in terms of simplicity of the current deal.II adapter.