Inconsistent definition of poly_degree in MultiIndexSet

Given a non-complete multi-index set to create a MultiIndexSet, the polynomial degree is inconsistently computed.

The Issue

Here's a reproducible example for this:

import numpy as np
import minterpy as mp

# An incomplete multi-index set
xx = np.array([
  [0, 0],
  [1, 0],
  [0, 1],
  [0, 2],
  [1, 3]
])
mi = mp.MultiIndexSet(xx, lp_degree=1.0)

print(mi.poly_degree)  # 3
print(mp.core.utils._get_poly_degree(xx, lp_degree=1.0)  # 3
assert mi.poly_degree == mp.core.utils._get_poly_degree(xx, 1)

This is because the computation of the polynomial degree as an attribute of MultiIndexSet is simply taken as the maximum exponent (independent of the lp-degree) and therefore not consistent with how the degree should be computed (i.e., using the internal function mp.core.utils._get_poly_degree that depends on the lp-degree as well). The maximum exponent as degree is only valid for lp-degree equals to inf.

This also matters because if an incomplete multi-index set is made complete (via the method make_complete()), then the polynomial degree of the complete set has a different polynomial degree.

print(mi.poly_degree)  # 3
print(mi.make_complete().poly_degree)  # 4

I consider this as an unexpected behavior as well.

Another downstream issue found during the merging of the current dev branch to the optimiser_refac branch is related to Grid. Making a Grid complete is making the underlying multi-index complete. But because the polynomial degree of a Grid is the polynomial degree of the underlying multi-index set, making a Grid complete and changing inadvertently the polynomial degree will fail because:

A grid of <grid.poly_degree> cannot consist of indices with degree <grid.multi_index.poly_degree>

Test coverage

Creating an incomplete multi-index set (with or without making it complete afterward) is not yet covered in the current test suite.

A Solution

Use the internal utility function mp.core.utils._get_poly_degree to compute the polynomial degree of a MultiIndexSet.