Rename DamagedElasticEnergyDensityWrapper to IntegralDensityWrapper

It is a general type erasure wrapper around integral densities, and there is (almost) nothing damage-specific to it. Therefore, we rename the class and change the template parameters: The previous parameter was the domain dimension, which would be used to compute the correct jet size for the brittle-fracture problem. This was the one thing about the class that was specific to fracture mechanics. To get rid of it the new implementation gets the jet implementation class as the template parameter instead.