Support native cloning of Base objects

[clue]

Support native cloning of Graph, Vertex and Edge.

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The vertex ID is currently unique within the whole graph. This means that we can not create an exact clone of the vertex within the same graph. This is what currently blocks this ticket.

In the linked PR #89 @clemens-tolboom suggested assigning a new vertex ID when cloning a Vertex instance. This would allow us to work around this limitation. I'm not perfectly happy with this tbh, but perhaps this is really only a cosmetic issue if done right.

As an alternative, ticket #131 questions whether we actually need the vertex ID in the first place and suggests dropping it altogether. Voilà, cloning made easy.

Any input would be welcome!

[clue]

Solving this isn't too hard from a technical perspective, but I've decided against putting any more time into this simply because its API is somewhat unclear. Take the following example:

$graph = new Graph();
$v1 = $graph->createVertex();
$v2 = clone $v1;

assert(count($graph->getVertices()) === 2); // ???

I don't think the suggest behavior makes a lot of sense, so I really don't think native cloning provides a sufficiently reasonable API. the existing API methods such as createVertexClone() make this behavior explicit 👍