PMP Feature request: Add longest_edge()

[afabri]

Issue Details

Add a function that returns a halfedge descriptor for the longest edge of a mesh. I am also surprised that we have a function computing the edge length and not the squared edge length. We should not call sqrt for all edges for finding the longest one.

[sloriot]

almost a oneliner with std::max_element() but I agree it is nice to have.