ctrl_panel: Limits fixes

* PID Autotune test current should be positive

* max_v should be 4 V not 5 V

* r0 should not be negative

pytec/pytec/gui/view/param_tree.json

@@ -156,7 +156,7 @@
                      "step": 0.1,
                      "limits": [
                         0,
-                        5
+                        4
                      ],
                      "siPrefix": true,
                      "suffix": "V",
@@ -209,6 +209,7 @@
                "type": "float",
                "value": 10000,
                "step": 1,
+               "min": 0,
                "siPrefix": true,
                "pinSiPrefix": "k",
                "suffix": "Ω",
@@ -402,7 +403,7 @@
                      "decimals": 6,
                      "step": 0.1,
                      "limits": [
-                        -2,
+                        0,
                         2
                      ],
                      "siPrefix": true,