Fix interfacename connection setting

[kwokcb]

Interface Connection Setting

Fixes #59

This change fixes the issue of creating both an interfacename connection and also setting a value on an input
when serizliating the input on a node in a graph. Only one should be specified to be valid.

e.g. Currently you get this for example which is invalid for "in`" on the "Add" node:

<nodegraph name="Nodegraph" xpos="-1.7653979153374229" ypos="3.8192409855171383">
    <add name="Add" type="float" xpos="3.9042865959028106" ypos="2.7340644554414775">
      <input name="in1" type="float" interfacename="in_Add" value="0" />
      <input name="in2" type="float" value="0" />
    </add>
    <output name="out_Add" type="float" nodename="Add" />
    <input name="in_Add" type="float" value="0" />
  </nodegraph>

This fix remove the value attribute to get this instead

<nodegraph name="Nodegraph" xpos="-1.7653979153374229" ypos="3.8192409855171383">
    <add name="Add" type="float" xpos="3.9042865959028106" ypos="2.7340644554414775">
      <input name="in1" type="float" interfacename="in_Add" />
      <input name="in2" type="float" value="0" />
    </add>
    <output name="out_Add" type="float" nodename="Add" />
    <input name="in_Add" type="float" value="0" />
  </nodegraph>

Note that nodename and nodegraph settings are okay since the Python calls being made explicitly
remove the value attribute for you.

[manuelkoester]

Perfect, thank you Bernard!