Add asset model node type for Method

[wazzamatazz]

Adding a new Method field to the NodeType enum would make it easier to represent method nodes in OPC UA address spaces.

[wazzamatazz]

Note that adding new fields to an enum would cause serialization/deserialization compatibility issues with older clients. Need to assess if thise would be worthwhile.

[wazzamatazz]

Closing this ticket. This can be represented using a combination of NodeType and a sub-type when creating a node.