The method doesn't make sense yet, as `Triangle::new` explicitly forbids
degenerate triangles. This is about to change though, and adding this
method is preparation for that.

This check is currently redundant with what the `Triangle` constructor
already does. The constructor will soon get simplified though, and then
the new check will be necessary.

Degenerate triangles are actually useful in some cases, and I'm actually
working on some code that needs them. Thus, the constructor as it was,
was too restrictive.

With the constructor no longer preventing degenerate triangle, there's
no reason to keep it private.

With the `points` field public, it has become redundant.