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

Following Issues #103 (closed) and #117 (closed), the method expand_dim() of the MultiIndexSet class shall also be refactored to include an inplace parameter to have a more transparent behavior after expanding the dimension of an existing multi-index set instance. Currently, when this method is called, the modification happens strictly in-place; if a multi-index set is attached to an instance of a Grid or one of the polynomial classes, then calling the method will change all the multi-index set attached to those instances. This may create unexpected behavior downstream.

Following the other two methods (i.e., add_exponents() and make_complete()), the default behavior should be that a new instance is created when the dimension is expanded. Only if inplace is set to True, the modification happens in-place.

	dimension not-expanded	dimension expanded
inplace=False	return a shallow copy	return a new instance
inplace=True	instance unmodified	instance modified

The dimension may not be expanded if the value of the dimension argument (i.e., m in expand_dim(m)) is the same as the dimension of the current multi-index set instance.

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

Furthermore, this is part of refactoring the code in-line with Issue #116, where polynomials arithmetics (addition, subtraction, multiplication) becomes important. The expansion of multi-index set is used during addition, subtraction, and multiplication of two polynomials with differing spatial dimensions.