Define an empty MultiIndexSet instance

To further support the set operations involving instances of MultiIndexSet, the empty MultiIndexSet instance shall be defined.

An empty MultiIndexSet can be constructed with:

>>> import minterpy as mp
>>> mi = mp.MultiIndexSet(np.array([[]]), lp_degree=1.0)
>>> mi
MultiIndexSet
[]
>>> mi.spatial_dimension  # empty set may have 0 spatial dimension
0
>>> mi = mp.MultiIndexSet(np.empty((0, 5)), lp_degree=2.0)
>>> mi
MultiIndexSet
[]
>>> mi.spatial_dimension  # empty set may have its own spatial dimension
5
>>> mi.poly_degree  # poly. degree is `None` as it can't be computed
>>>

Note that, to create an empty MultiIndexSet an empty NumPy two-dimensional array must be fed to the constructor. Furthermore, the parameter lp_degree remains obligatory.

The following properties are defined a priori for an empty set:

• exponents (_exponents): a two-dimensional empty array as specified.
• lp_degree (_lp_degree): as specified.
• poly_degree (_poly_degree): None.
• spatial_dimension (_spatial_dimension): according to the dimension of the empty array.
• is_complete (_is_complete): False.
• is_downward_closed (_is_downward_closed): False.

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