Additional Possibilities for I_RunOps('save_integrals_over_time')

[ranocha]

Sometimes, secondary integral quantities such as the total kinetic energy or entropy can be of interest, e.g. in https://arxiv.org/abs/1604.06618. It would be nice to add a possibility to compute them, depending on the model and test case at hand.

As far as I know, the integral quantities of interest depend mostly on the physical model, e.g. ideal_gas_Euler or induction_equation. Should we split I_RunOps('save_integrals_over_time') into I_RunOps('save_integrals_over_time') (energy, entropy, ...) and I_RunOps('save_errors_over_time') (as it is now)? Then, I_RunOps('save_integrals_over_time') can depend on the physical model.

What do you think, @philipheinisch, @Kostaszki?