Added some optimization to `SolidMechanicsModel::reassignMaterial()`.
The function now checks if the assignement of a material has changed and only in that case calls Material::addElements()
and Material::removeElements()
.
Further the same optimization was applied to the mentioned material functions.
In addition this commit also fixed am error related to the debug system (a missing AKANTU_DEBUG_OUT()
call was missing in Material::removeElements()
).