support 'if/else' on constant arguments

The following graph excerpt currently fails:

 "eq6": {
    "process_id": "eq",
    "arguments": {
      "x": {
        "from_parameter": "region"
      },
      "y": "Deserts"
    }
  },
  "if3": {
    "process_id": "if",
    "arguments": {
      "accept": {
        "from_node": "divide13"
      },
      "reject": 0,
      "value": {
        "from_node": "eq6"
      }
    }
  },

Problem is that we only support the more complex case, where the condition is applied to raster values.
The simple case where the parameter value is a constant (from a udp) which is compared to another constant is not supported yet.
It may be easier to handle this in the python side, and thus achieve an optimization where only the chosen branch of the if statement is evaluated further.