Fix nutils-nutils heat transport case

[BenjaminRodenberg]

#148 applies the new structure from #125 to the heat transport case. We also want to be able to run the heat transport case using nutils-nutils or any combination of fenics and nutils.

I will create a PR with a prototype implementation in a moment.

[BenjaminRodenberg]

I'll try to fix this issue until Apr. 21.

[MakisH]

@BenjaminRodenberg is this still open after having merged #153?

[BenjaminRodenberg]

Issue is solved, since #153 is merged.