add more examples in the docstrings

[franaudo]

Include simple implementation examples directly in the doc strings, like:

    Parameters
    ----------
    xyz : list
        [x, y, z] co-ordinates of the node.
    x : float
        x co-ordinates of the node.
    y : float
        y co-ordinates of the node.
    z : float
        z co-ordinates of the node.
    ex : list
        Node's local x axis.
    ey : list
        Node's local y axis.
    ez : list
        Node's local z axis.
    mass : float
        Mass in kg associated with the node.
    label : string
        Node's label. If no label is specified, it is automatically generated
        when a node is added. The label does not need to be unique.
    key : int
        Node key number. The key number is unique.

    Examples
    --------
    >>> node = Node(1.0, 2.0, 3.0)
    """