Comparing multipatch domains with == raises AttributeError #197

Comparing two multipatch Domain objects with == raises an AttributeError, because MultiPatchMapping has no _name attribute, which Mapping._hashable_content uses.

Reproduced with SymPDE 0.20.0:

from sympde.topology import Domain, Mapping, Square

A = Mapping('FA', dim=2)(Square('A'))
B = Mapping('FB', dim=2)(Square('B'))
D1 = Domain.join([A, B], [((0, 0, 1), (1, 0, -1),  1)], 'Omega')
D2 = Domain.join([A, B], [((0, 0, 1), (1, 0, -1), -1)], 'Omega')

D1 == D2
# AttributeError: 'MultiPatchMapping' object has no attribute '_name'

Expected

Here == should return False, since the interface orientations differ.

Context

In PSYDAC we need to check that a user-given domain matches the domain stored in a geometry file. We currently work around this by comparing patches and interfaces one by one.