Replace local energies by local densities

• wrap the densities by a generic IntegralEnergy
• this preserves all material properties
• reduces redundant code

dune/elasticity/materials/CMakeLists.txt

 install(FILES
     adolcmaterial.hh
     adolcneohookeanmaterial.hh
+    exphenckydensity.hh
     exphenckyenergy.hh
     geomexactstvenantkirchhoffmaterial.hh
+    henckydensity.hh
     henckyenergy.hh
+    localdensity.hh
+    localintegralenergy.hh
+    mooneyrivlindensity.hh
+    mooneyrivlinenergy.hh
     neohookeanmaterial.hh
+    neohookedensity.hh
     neohookeenergy.hh
     neumannenergy.hh
     ogdenmaterial.hh
+    stvenantkirchhoffdensity.hh
     stvenantkirchhoffenergy.hh
     sumenergy.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/elasticity/materials)