Implement a method to make a Grid instance downward-closed.

A new method make_downward_closed() is introduced for instances of the Grid class. If the current instance is not downward-closed, then a downward-closed set with respect to the current multi-index set is first created and a new instance of Grid is created.

Additionally:

• A new property is_downward_closed() is available for Grid instances; it carries over from the underlying MultiIndexSet a property of the same name.
• The test suite has been updated to reflect the current changes.
• A test fixture is used to generate non-downward-closed over different spatial dimensions, polynomial degrees, and lp-degrees.
• A How-To Guide demonstrating the usage of make_downward_closed() is added to the docs.

This commit should resolve Issue #156 (closed) and part of the refactoring and update of the Grid class (Issue #135).