Fix Issue #103: Refactor make_complete() of MultiIndexSet

• The flag parameter inplace has been added to the make_complete() method of the MultiIndexSet class. When the parameter is set to True, the modification to underlying exponents will be made in-place; when False (the default), a new instance with a complete exponents will be created.

This MR should resolve Issue #103 (closed). The creation of new instance after the modification of the underlying set of exponents is made explicit by the inplace parameter. The default value of False preserves the previous behavior of calling the function.