make the ECFV simulator for the Richards-lens problem use AD

for the Richards model we can't use the CO2 injection problem because
this problem cannot be simulated by the Richards model. (Well,
strictly speaking the Richards model *can* simulate it, but it would
only produce garbage because the assumptions of the Richards model are
violated by that problem.)

examples/lens_richards_ecfv.cpp

@@ -32,6 +32,9 @@ namespace Ewoms {
 namespace Properties {
 NEW_TYPE_TAG(RichardsLensEcfvProblem, INHERITS_FROM(RichardsLensProblem));
 SET_TAG_PROP(RichardsLensEcfvProblem, SpatialDiscretizationSplice, EcfvDiscretization);
+
+//! Use automatic differentiation to linearize the system of PDEs
+SET_TAG_PROP(RichardsLensEcfvProblem, LocalLinearizerSplice, AutoDiffLocalLinearizer);
 }}
 
 int main(int argc, char **argv)