Added Nusselt number computation based on thermal and kinetic dissipation

[brownbaerchen]

The Nusselt number describes the ratio of convective heat flux to conductive heat flux. Computing it in different fashions and making sure all values agree is a common test in verification of RBC codes.
You can either compute it directly from the temperature profile, or, using some energy conservation considerations, from dissipation rates of temperature and kinetic energy. We had the computation from the temperature profile before, this PR adds the latter two versions for additional comparison.

Furthermore, this PR includes a few small things:

• More debug output in spectral methods
• Better error handling in FieldsIO
• Page size for the Nature computational science TeX template
• Minor refactors

[tlunet]

Looks good to me, let's merge as soon as all tests pass (except the firedrake one ...)