Since NodeView and EdgeView ultimately inherit from collections.abc.Set, the user should be able to perform set operations on them. As reported in #206 this was not the case. This PR fixes that.

New functionality:

    H = hyperwithattrs
    nodes1 = H.nodes.filterby_attr("color", "blue")
    nodes2 = H.nodes.filterby("degree", 2, "geq")
    assert set(nodes2 - nodes1) == {3, 4}
    assert set(nodes1 - nodes2) == set()
    assert set(nodes1 & nodes2) == {2, 5}
    assert set(nodes1 | nodes2) == {2, 3, 4, 5}
    assert set(nodes1 ^ nodes2) == {3, 4}

Before this PR, any of the set operations would raise an exception.

The fix was easy. Just had to override collections.abc.Set._from_iterable to play well with the rest of the class.

Also added mention of this to the Tutorial.