Add support for the inplace multiplication operator for instances of MultiIndexSet

Following Issue #119 (closed) and MR !80 (merged), instances of MultiIndexSet may be multiplied via the * operator. For completeness of behaviors associated with the instances of the MultiIndexSet class, the inplace multiplication operator (*= via the method __imul__()) shall also be implemented such that:

>>> import minterpy as mp
>>> import numpy as np
>>> mi_1 = mp.MultiIndexSet.from_degree(2, 2, 1)
>>> mi_1
MultiIndexSet
[[0 0]
 [1 0]
 [2 0]
 [0 1]
 [1 1]
 [0 2]]
>>> mi_2 = mp.MultiIndexSet.from_degree(2, 1, 1)
>>> mi_2
MultiIndexSet
[[0 0]
 [1 0]
 [0 1]]
>>> mi_1 *= mi_2  # equivalent to mi_1 = mi_1 * mi_2
>>> mi_1
MultiIndexSet
[[0 0]
 [1 0]
 [2 0]
 [0 1]
 [1 1]
 [0 2]]
>>> mi_1.poly_degree
2

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