Guards for spatial dimension in MultiIndexSet `from_degree` initialization

Currently, the from_degree initialization for MultiIndexSet checks if the spatial_dimension is integer. However, it doesn't check if the integer spatial dimension is indeed valid. I think that spatial_dimension must be a strictly positive integer.

Currently,

import minterpy as mp
mi = mp.MultiIndexSet.from_degree(0, 2)

throws an exception from get_exponent_matrix function which is not very informative:

ValueError: at least one array or dtype is required

We can implement this guard for spatial_dimension in the from_degree class method or in the get_exponent_matrix function or perhaps in both places.