[components][air] Fix heat capacity

See merge request !601

Gas heat capacity was wrong by 3 orders of magnitude

[2pnc] fix calculation of enthalpy

See merge request !585

So far, no enthalpy has been calculated in the 2pnc(ni) model.


(cherry picked from commit a1109fbe)

Fix/tutorial for dune24

See merge request !579

[cmake] Add a build_tutorials convenience target to build all tutorials

See merge request !561

[bin] improve documentation of extract script

See merge request !558

Feature/dumux course corrections

See merge request !559

Dumux course tasks

See merge request !553