Prepare implementation of Tresca flow: make dissipation classes more generic

In this commit we introduce the class VonMisesDissipationDensity and use it as a template parameter for the (renamed) dissipation classes.

This prepares the implementation for the Tresca flow rule.

To avoid duplicated code in the future, the characteristic parts of the functional (value, gradient, hessian, subdiff, domain) are implemented directly in the dissipationDensity.

Since the localSolver will be completely different for the Tresca flow, we use a template specialization here.