Mixer node

Created mixer node for two audio sources. 

- Only tested on two oscillator nodes operating at different frequencies so function may need adjusting in the future (mixer_node.ls - Ln 29-30)
- Value ranges from 0 to 1 where 0 is wholly first input, and 1 is wholly second.
- Execute tests if you like by adding appropriate imports at the top, and maybe this:
   - @o1 = new Oscillator [500px 400px], @actx
    -  @o1.receive-for-ref 1, 1
    -  @o2 = new Oscillator [500px 600px], @actx
    -  @o2.receive-for-ref 1, 0
    -  @mn = new MixerNode [600px 500px], @actx
    -  @mn.receive-for-ref 1, 0
    -  @w1 = new Wire @o1
    -  @w2 = new Wire @o2
    -  @w3 = new Wire @mn
    -  @w1.connect @mn
    -  @w2.connect @mn
    -  @w3.connect destination