add more examples in the docstrings

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)
"""
```