Estimates of wave speeds

Often, we need some estimates of the wave speeds, e.g.

- for CFL-based time step control
- for numerical fluxes like local Lax-Friedrichs, Harten-Lax-van Leer

In principle, multiple estimate of the wave speeds can be used, e.g.

- "naive" values based on the local wave speeds of one or two nodes
- exact value from the solutions of the corresponding Riemann problem (if available)
- some estimates from the literature

~Can/shall we try to create some unified interface for these estimates?~
We created a unified interface for wave speed estimates in #493. Now, we shall to add more sophisticated wave speed estimates, e.g.
- [ ] Euler euations
- Guermond and Popov (2016) Fast Estimation from above of the maximum wave speed in the Riemann problem for the Euler equations, Journal of Computational Physics 321, pp. 908-926
- Chen and Shu (2017) Entropy stable high order discontinuous Galerkin methods with suitable quadrature rules for hyperbolic conservation laws, Journal of Computational Physics 345, pp. 427-461