add_exponents in MultiIndex returns a multi-index set of different lp-degree

The method add_exponents of a MultiIndexSet instance may return a new instance that is of different lp-degree.

Here's a reproducible example:

import minterpy as mp
import numpy as np

mi = mp.MultiIndexSet.from_degree(3, 2, 1)   # lp-degree = 1

mi_added = mi.add_exponents(np.array([0, 0, 3]))  # lp-degree becomes 2 (the default)

assert mi.lp_degree == mi_added.lp_degree

This is an unexpected behavior.

The method add_exponents itself has not been covered in the unit test. So it's a good idea to expand the unit test.

Thanks to @schrei06 who indicates the issue.