    cellView.findPortNode(portId)

Return the port element of this CellView that is identified by `portId` (i.e. the SVGElement within `this.el` which has `'port': portId`).

If the CellView does not have a port identified by `portId`, return `null`.

    cellView.findPortNode(portId, selector)

If `selector` is also specified, return the subelement of the port element that is identified by `selector`, according to the rules of the `cellView.findBySelector()` function.

If the port element does not have a subelement that matches `selector`, return `null`.