fix bunge et al cookbook

fixes

```
An error occurred in line <261> of file
</home/heister/aspect-git/source/material_model/depth_dependent.cc> in
funct
ion
void
aspect::MaterialModel::DepthDependent<dim>::parse_parameters(dealii::ParameterHandler&)
[with int dim = 2]
The violated condition was:
reference_viscosity != std::numeric_limits<double>::max()
Additional information:
    You have to set a reference viscosity for the depth dependent model.
 ```

Author: tjhei

cookbooks/bunge_et_al_mantle_convection/bunge_et_al.prm

@@ -14,6 +14,7 @@ subsection Material model
     set Depth dependence method = File
     set Data directory = $ASPECT_SOURCE_DIR/cookbooks/bunge_et_al_mantle_convection/
     set Viscosity depth file = visc_depth_a.txt
+    set Reference viscosity = 1e22
   end
   subsection Simple model
     set Reference density             = 4500