Use a set for upward connectivity? #3
Description
In bempp/ndgrid#111, a TODO about this in the code was removed.
In experiments so far, replacing the innermost Vec in https://github.com/bempp/ndgrid/blob/a1a33b18b2e58cadbf86209b6e39a9d6eca9eb74/src/topology/single_type.rs#L23 with a HashSet (see bempp/ndgrid@03db90d) was not successful, as the upwards connectivity currently needs to be in order.