Check that a `pp.Domain` is simply connected

[jhabriel]

Checks for whether a domain is simply-connected should be added to the pp.Domain class. This should be straightforward for 2d, but it is slightly more involved for 3d.

Currently, this is taken as an assumption. However, no proper check is in place.

[keileg]

I agree we should ideally fix this, but is it really an issue which should be prioritized over other problems?

[jhabriel]

I would say it has a low priority, yes. I will add the the label.