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.

Edited by Praetorius, Simon

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