Refactor add_points() method of the Grid class.

The method add_points() of the Grid class has been renamed to add_exponents() to be consistent with a method of the same name of the MultiIndexSet class. Calling the method on an instance with a set of exponents adds the exponents to the underlying multi-index set and returns a new instance of Grid with the updated multi-index set.

Additionally:

• The test suite has been updated to reflect the current changes.
• A new How-To Guide that demonstrates the method usage is added to the docs.

The commit should resolve Issue #158 (closed) and part of the refactoring and update of the Grid class (see Issue #135).