Refactor the method to make a Grid instance complete.

The method make_complete() has been refactored to always return a new instance of Grid. If the current instance is not complete, then a complete multi-index set is first created a new instance of Grid is then created based on the new multi-index set.

Additionally:

• A new property is_complete() 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 How-To Guide demonstrating the usage of make_complete() is added to the docs.

This commit should resolve Issue #157 (closed) and part of the refactoring and update of the Grid class (see Issue #135).