ctrl_panel: Limit fixes

* PID Autotune test current should be positive
* Maximum absolute voltage should be 4 V not 5 V

pytec/pytec/gui/view/param_tree.json

@@ -141,7 +141,7 @@
                      "step": 0.1,
                      "limits": [
                         0,
-                        5
+                        4
                      ],
                      "siPrefix": true,
                      "compactHeight": false,
@@ -365,7 +365,7 @@
                      "compactHeight": false,
                      "step": 100,
                      "limits": [
-                        -2000,
+                        0,
                         2000
                      ],
                      "pid_autotune": [