ctrl_panel: Use ADC sample interval as PID unit

The Thermostat PID gains are actually in units relative to the sampling
interval of the Thermostat, and not SI seconds. Reflect that accordingly
in the units of PIDs.

See https://hackmd.io/IACbwcOTSt6Adj3_F9bKuw#Units for more details.

pytec/pytec/gui/view/param_tree.json

@@ -290,7 +290,7 @@
                "title": "Ki",
                "type": "float",
                "step": 0.1,
-               "suffix": "Hz",
+               "suffix": "τ⁻¹",
                "noUnitEditing": true,
                "compactHeight": false,
                "thermostat:set_param": {
@@ -305,7 +305,7 @@
                "title": "Kd",
                "type": "float",
                "step": 0.1,
-               "suffix": "s",
+               "suffix": "τ",
                "noUnitEditing": true,
                "compactHeight": false,
                "thermostat:set_param": {