Updates required for QAT

This MR includes:

• Abs operator;
• Hook system for Node (as suggested in #98 (closed) @thibaultallenet).
• Update of SinglePassGraphMatching to match no connection (with the special node keyword $).
• Solved some SinglePassGraphMatching issues and added more complex unit tests.
• Added missing inChannels() method for FC operator;
• Added matchFrom() method to SinglePassGraphMatching in order to match a query only for a specific starting node;
• Fix issue #87 (closed).