Fix equality, hash and is_reference of PhysicalToReference - #88

PhysicalToReference.__eq__ checks isinstance(other, ReferenceToPhysical), and __hash__ uses the key "uflx.ReferenceToPhysical": both were copied from ReferenceToPhysical. As a result:

• a PhysicalToReference is not equal to itself, or to a copy rebuilt from the same arguments, which is how replace and the pull-back rebuild nodes, so a rebuilt copy is not found in a dict keyed by the original;
• it is equal to the ReferenceToPhysical with the same arguments, and has the same hash, so a set or graph merges the two opposite maps into one node.

It also does not override is_reference, so a point mapped to the reference cell reports False.

"""PhysicalToReference compares, hashes and reports itself as a ReferenceToPhysical."""

from basix_uflx import element
from uflx import coordinate_element
from uflx.expressions import RealScalar
from uflx.geometry import PhysicalToReference, ReferenceToPhysical
from uflx.points import Point

domain = coordinate_element(element("Lagrange", "triangle", 1, shape=(2,)))
x = Point([RealScalar(0.1), RealScalar(0.2)], is_reference=True)

G = PhysicalToReference(x, domain)
F = ReferenceToPhysical(x, domain)
print("G == G:", G == G)
print("G == PhysicalToReference(x, domain):", G == PhysicalToReference(x, domain))
print("G == ReferenceToPhysical(x, domain):", G == F)
print("len({G, ReferenceToPhysical(x, domain)}):", len({G, F}))
print("G.is_reference:", G.is_reference)

returns on main

G == G: False
G == PhysicalToReference(x, domain): False
G == ReferenceToPhysical(x, domain): True
len({G, ReferenceToPhysical(x, domain)}): 1
G.is_reference: False

and with this PR

G == G: True
G == PhysicalToReference(x, domain): True
G == ReferenceToPhysical(x, domain): False
len({G, ReferenceToPhysical(x, domain)}): 2
G.is_reference: True

Changes

• PhysicalToReference.__eq__ and __hash__ refer to PhysicalToReference.
• PhysicalToReference.is_reference is True.
• New test/test_point_map_equality.py. Both tests fail on main.