Remove `exponents_completed` property of the `MultiIndexSet` class

The read-only property exponents_completed is used to store the set of exponents that is complete with respect to the given spatial dimension, polynomial degree, and lp-degree. Because of the lazy evaluation, the complete set of exponents will be computed (and stored) only when the property is accessed. It is potentially expensive to store the completed set of exponents in a separate property. I've not seen a use case for accessing this property in the code base and, most probably, anywhere else. I propose to delete this property altogether to simplify the class.

Instead, when needed, the method make_complete() should be used to create a complete set of exponents when needed. Using the refactored method (see Issue #103 (closed)), users have two choices: create a new multi-index set with the completed exponents or modify the current exponents in-place. There is no need to store the exponents separately in a dedicated property.

This issue is related to the refactoring of MultiIndexSet (see Issue #99 (closed)).