Add coefficient type to ElementMatrix and ElementVector

Try to allow different coefficient types than double, e.g. float or Dune::Float128. The Test with Float128 requires some changes in dune. So, it is skipped here.

I'm not sure about the design: How to specify the type of the coefficients (element matrix/vector type)? Currently, I have just added a CoefficientType member type to the ProblemStatTraits class. Maybe we should discuss to define the backend of DOFMatrix and DOFVector there and derived the coefficient type from these container classes.