Support getting nodes number and edge number methods for Graph.

Add the following methods for `Graph`:
```python

def number_of_nodes(label=None):
"""Return the number of nodes under a specified label.
If label is None, then return the number of nodes of the whole graph.
"""
pass

def number_of_edges(label=None):
"""Return the number of nodes under a specified label.
If label is None, then return the number of edges of the whole graph.
"""
pass

```