Pattern match context

[gramalingam]

• Introduce a class called PatternMatchContext to capture information about the context describing a match to a given pattern, containing the information described below.
• Create an instance of this class at this line and pass it to the condition-function called later.

The class PatternMatchContext should be primarily a read-only API with the following:

• a property representing the model being matched,
• a property representing the graph or function being matched,
• a property representing the main root node of the matching subgraph
• a property representing the output values of the matching subgraph (a sequence)
• a property representing all the nodes of the matching subgraph

The class should have a constructor that accepts the model, graph/function, main root node, and a MatchResult.