Overwrite gantry height value instead of default_value

Because the default in fdmprinter has a formula in the value, the default_value properties wouldn't get used any more.

resources/definitions/101Hero.def.json

@@ -38,7 +38,7 @@
         "speed_wall": { "value": "speed_print * 0.7" },
         "speed_topbottom": { "value": "speed_print * 0.7" },
         "speed_layer_0": { "value": "speed_print * 0.7" },
-        "gantry_height": { "default_value": 0 },
+        "gantry_height": { "value": "0" },
         "retraction_speed": { "default_value" : 10 },
         "retraction_amount": { "default_value" : 2.5 },
         "machine_gcode_flavor": { "default_value": "RepRap (Marlin/Sprinter)" },

resources/definitions/3dator.def.json

@@ -45,7 +45,7 @@
             ]
         },
         "gantry_height": {
-            "default_value": 30
+            "value": "30"
         },
         "machine_start_gcode": {
             "default_value": ";Sliced at: {day} {date} {time}\nM104 S{material_print_temperature} ;set temperatures\nM140 S{material_bed_temperature}\nM109 S{material_print_temperature} ;wait for temperatures\nM190 S{material_bed_temperature}\nG21  ;metric values\nG90  ;absolute positioning\nM82  ;set extruder to absolute mode\nM107  ;start with the fan off\nG28 Z0  ;move Z to min endstops\nG28 X0 Y0  ;move X/Y to min endstops\nG29  ;Auto Level\nG1 Z0.6 F{speed_travel} ;move the Nozzle near the Bed\nG92 E0\nG1 Y0  ;zero the extruded length\nG1 X10 E30 F500  ;printing a Line from right to left\nG92 E0  ;zero the extruded length again\nG1 Z2\nG1 F{speed_travel}\nM117 Printing...;Put printing message on LCD screen\nM150 R255 U255 B255 P4 ;Change LED Color to white"      },

resources/definitions/Mark2_for_Ultimaker2.def.json

@@ -37,7 +37,7 @@
             "default_value": 203
         },
         "gantry_height": {
-            "default_value": 52
+            "value": "52"
         },
         "machine_center_is_zero": {
             "default_value": false

resources/definitions/alfawise_u20.def.json

@@ -39,7 +39,7 @@
             "default_value": false
         },
         "gantry_height": {
-            "default_value": 10
+            "value": "10"
         },
         "machine_gcode_flavor": {
             "default_value": "RepRap (Marlin/Sprinter)"

resources/definitions/alfawise_u30.def.json

@@ -36,7 +36,7 @@
         "retraction_enable": { "default_value": true },
         "retraction_amount": { "default_value": 5 },
         "retraction_speed": { "default_value": 45 },
-        "gantry_height": { "default_value": 25 },
+        "gantry_height": { "value": "25" },
         "machine_width": { "default_value": 220 },
         "machine_height": { "default_value": 250 },
         "machine_depth": { "default_value": 220 },

resources/definitions/alya3dp.def.json

@@ -30,7 +30,7 @@
         "machine_height":              {   "default_value": 133             },
         "machine_depth":               {   "default_value": 100             },
         "machine_center_is_zero":      {   "default_value": false           },
-        "gantry_height":               {   "default_value": 55             },
+        "gantry_height":               {   "value": "55"},
         "retraction_amount":           {   "default_value": 1.5 },
         "support_enable":              {   "default_value": true},
         "machine_head_with_fans_polygon": {

resources/definitions/alyanx3dp.def.json

@@ -30,7 +30,7 @@
         "machine_height":              {   "default_value": 170             },
         "machine_depth":               {   "default_value": 160             },
         "machine_center_is_zero":      {   "default_value": false           },
-        "gantry_height":               {   "default_value": 55             },
+        "gantry_height":               {   "value": "55"},
         "retraction_amount":           {   "default_value": 1.5 },
         "support_enable":              {   "default_value": true},
         "machine_head_with_fans_polygon": {

resources/definitions/anycubic_4max.def.json

@@ -50,7 +50,7 @@
         "jerk_wall": { "value": "math.ceil(jerk_print * 10 / 25)" },
         "jerk_wall_0": { "value": "math.ceil(jerk_wall * 5 / 10)" },
 
-        "gantry_height": { "default_value": 25.0 },
+        "gantry_height": { "value": "25.0" },
         "skin_overlap": { "value": "10" },
 
         "acceleration_enabled": { "value": "True" },

resources/definitions/anycubic_chiron.def.json

@@ -52,7 +52,7 @@
         },
         "gantry_height":
         {
-            "default_value": 35
+            "value": "35"
         },
         "machine_head_with_fans_polygon":
         {

resources/definitions/anycubic_i3_mega.def.json

@@ -46,7 +46,7 @@
         },
         "gantry_height":
         {
-            "default_value": 0
+            "value": "0"
         },
         "machine_gcode_flavor":
         {