Refactor add_points() method for Grid instances

The instance method add_points() of the Grid class shall be renamed to add_exponents() to avoid confusion because what is being added is actually exponents of multi-index set, not points in the grid. The method add_exponents() is a carry-over method of the same name from the underlying MultiIndexSet instance attached to the instance of Grid. Currently, calling the method will return an instance of Grid whose multi-index set is updated with the new exponents; the returned instance, however, may or may not be identical with the current instance. Instead of this implicit behavior, it would be better that calling the method always return a new instance by default. In principle, an in-place parameter may be introduced to the method to allow for an explicit in-place modification.

Similar carry-over methods from the underlying MultiIndexSet instance are make_compete() (Issue #157 (closed)) and make_downward_closed() (Issue #156 (closed)).

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).