Add support for multioutput connection edits

[kwokcb]

Issue

Fixes #1469

Incorrect connections were being stored on the MaterialX document when making / breaking connections with upstream nodes which have more than one output (mulitoutput nodes).

Fixes

• Make sure to remove any reference on an input to an upstream output attribute when breaking links.
• Make sure to add the connection to an upstream output if the upstream node is multioutput. Otherwise use the current logic to connect to the node.
• If we always add an output reference on the downstream input, then there is a validation warning if the upstream node is not multioutput but you specify an output on the downstream input. This might be worthwhile to remove as it's not really an error.

Tests

Tested with issue graph:

<?xml version="1.0"?>
<materialx version="1.38">
  <separate3 name="separate3_vector3" type="multioutput" xpos="5.050725" ypos="-2.000000">
    <input name="in" type="vector3" nodename="constant_vector3" />
  </separate3>
  <constant name="constant_vector3" type="vector3" xpos="3.405797" ypos="-1.965517" />
  <combine3 name="combine3_vector3" type="vector3" xpos="7.094203" ypos="-2.034483">
    <input name="in1" type="float" nodename="separate3_vector3" output="outy" />
    <input name="in2" type="float" nodename="separate3_vector3" output="outx"/>
    <input name="in3" type="float" nodename="separate3_vector3" output="outz"/>
  </combine3>
</materialx>

• Test making and breaking connections on the combine node. (Should set / break at the output level).
• Test making and breaking connection on the constant. (Should be the same as before)

[lfl-eholthouser]

I tested it and it all looks great to me. Thanks for tackling this issue Bernard!

[jstone-lucasfilm]

Great work, thanks @kwokcb!