Numerical audio nodes

Created numerical_node.ls and audio_node.ls, both subclasses of Node.

Numerical Node
  - creates a maths node with 1 of each audio and numerical input
  - has a variable @value which is the contents of the node
  - has function send which sends value to its children
  - has function receive which receives a value from its parent
    - currently this simply propagates @value

Audio Node
  - creates an instrument node with 1 of each audio and numerical input
  - has a variable @source which is an audio node
  - overrides register-output(wire) to connect the audio node to its children
    - currently simply propagates the audio
  - overrides rem-output(node) to disconnect the audio node from a specific child