Change `explode` and `explode_layers` to use graph views instead of magic options on the edge ref

[ljeub-pometry]

The fact that edges have two ways of specifying time and layer bounds causes a bunch of weird inconsistencies, e.g.:

        let g = Graph::new();
        let e1 = g.add_edge(0, 1, 2, NO_PROPS, Some("1")).unwrap();
        let e2 = g.add_edge(1, 1, 3, NO_PROPS, Some("2")).unwrap();

        assert!(e1.has_layer("2"));
        assert!(e1.layers("2").unwrap().history().is_empty());

currently passes even though e1 is a view of the edge in layer "1"