Fix uggrid entityseeds

Previously, entity seeds only worked for elements and vertices.

mentioned in issue #22 (closed)

It seems to work with your fix. My test code:

for (auto const& e : elements(grid.leafGridView()))
{
    auto edge = e.subEntity<1>(0);
    auto s = edge.seed();
}

(The example code in the issue #22 (closed) tries to iterate over all edges. This seems not to be implemented, but the same error occurred for just one subEntity of an element)

mentioned in commit 7a228297

Status changed to merged

Mentioned in commit 7a228297