Remove `exponents_completed` property from MultiIndexSet.

The public property exponents_completed is removed from the MultiIndexSet class. This property is redundant and may cause large and unnecessary memory requirement of an instance of the class. The corresponding private property _exponents_completed is removed.

This MR should resolve Issue #104 (closed).

Several additional changes, mainly about cleaning up:

• Remove the yet implemented method ordering().
• Remove the private method _new_instance_if_necessary() as the updated overall implementations for some methods always return a new instance and only with a relevant parameter in-place modification is carried out.
• Remove the yet implemented dunder method __add__() as the addition operation does not make any sense for a set.
• Reorganize dunder methods position in the source code.
• Update in-code documentation and TODO's.