keeping track of underlying graph of the `Pattern`

[shinich1]

Is your feature request related to a problem? Please describe.
Instead of extracting the graph every time we do some operation on pattern, we can keep track of changes in the underlying graph by:

• having a property pattern.__graph
• update it in add method, or other methods that may change the graph shape

[thierry-martinez]

I think the current consensus is not to keep track of the underlying graph on the fly, but to have proper methods to check runnability (#364) and extract the graph on demand. Operations that need the graph are better defined on OpenGraph rather than Pattern.