Fix function call expression

[AntoineGautier]

The parser produces incorrect function call expression objects.

          "expression": {
            "operator": "function_call",
            "operands": [
              {
                "operator": "fill",
                "operands": [
                  {
                    "operator": "none",
                    "operands": [
                      "fill" // <--- Function name instead of function args
                    ]
                  },
                  {
                    "operator": "none",
                    "operands": [
                      "fill" // <--- Function name instead of function args
                    ]
                  }
                ]
              }
            ]
          },

whereas the original Modelica JSON contains:

                                    "expression": {
                                      "simple_expression": {
                                        "function_call": {
                                          "name": "fill",
                                          "arguments": [
                                            {
                                              "name": "0"
                                            },
                                            {
                                              "name": "3"
                                            }
                                          ]
                                        }
                                      }
                                    }