Add method makeActiveDensity to the LocalDensity interface

The method produces a copy of the density, but with field_type set to adouble. In the short-term, this is for use in dune-gfe only. There, the LocalIntegralEnergy class receives a density with any field_type, and replaces it by one that uses adouble. That way, the fact that ADOL-C is used for computing derivatives of the density is better hidden from the user.

Also, dune-gfe allows densities to implement their own derivatives, by-passing ADOL-C completely. I may implement this some day for dune-elasticity, too.

(cherry picked from commit c2dbf16f)