SortNestedHierarchies block should be migrated into the Model base class

[lucianopaz]

#76 made it apparent that nested hierarchies should be central to the Model object instead of being defined loosely across ModelBlocks. We should make the nested_hierarchies keyword part of the Model's signature, and then force the ModelBlocks to have nested_hierarchies that are in line with the enclosing model's hierarchy (i.e. blocks should specify the levels in a consistent order).