Implement make_downward_closed() method for Grid instances

The instance method make_downward_closed() of the Grid class shall be a carry-over method of the same name from the underlying MultiIndexSet instance attached to the instance of Grid. Similar carry-over methods from the underlying MultiIndexSet instance are add_exponents() (Issue #158 (closed)) and make_complete() (Issue #157 (closed)).

Calling make_downward_closed() on an instance of Grid creates a new instance whose underlying multi-index set is downward-closed. In principle, an in-place flag parameter may be introduced to the method to allow for an explicit in-place modification.

By making these methods available to the Grid instances, we can avoid directly working on the MultiIndexSet instances unnecessarily (and then create a new Grid instance) when dealing with instances of Grid.

This issue is part of the update and refactoring of the Grid class (Issue #135).