Refactor `add_exponents()` method of the `MultiIndexSet` class

Following Issue #103 (closed), the method add_exponents() of the MultiIndexSet class shall also be refactored to include inplace parameter to have a more transparent behavior after adding a set of exponents to an existing MultiIndexSet instance. Currently, when this method is called, we are not really sure whether the returned instance is an identical instance or a new instance that may lead to complication downstream.

The expected behaviors with respect to the MultiIndexSet instance are as follows:

	exponents unchanged	exponents changed
inplace=False	return a shallow copy	return a new instance
inplace=True	instance unmodified	instance modified

When an instance is modified in-place, the exponents are changed. Moreover, the poly_degree and is_complete (and later on, is_downward_closed, see Issue #105 (closed)) properties may also change.

A merge request related to this issue will also include an update to the in-code documentation of the method.

This issue is part of larger refactoring tasks in Issue #99 (closed).