Allow a Sud step to combine a ramp and a hold

A step can now carry both 'ramp' and 'hold': it ramps to the target
temp, then holds there for the given duration, instead of needing two
separate schedule entries. Sud.temp_reached() switches such a step
from its ramp phase into its hold phase in place (re-firing the
step-changed callback so SudTask/demo_sud re-apply the hold's stirrer
settings); user_wait_for_continue still applies once the whole step -
both phases - is done.

Merges sud_0010.json's three ramp/hold pairs into combined steps.

sude/sud_0010.json

@@ -54,20 +54,17 @@
 		{
 			"descr": "Einmaischen Rast",
 			"hold": {
-				"duration": 2000
+				"duration": 20
 			}
 		},
 		{
-			"descr": "Heizen Glucose-Rast",
+			"descr": "Glucose-Rast",
 			"ramp": {
 				"rate": 1.0,
 				"temp": 63
-			}
-		},
-		{
-			"descr": "Halten Glucose-Rast",
+			},
 			"hold": {
-				"duration": 4000,
+				"duration": 40,
 				"stirrer": {
 					"speed": 20,
 					"interval_time": 90,
@@ -76,16 +73,13 @@
 			}
 		},
 		{
-			"descr": "Heizen Verzuckerungs-Rast",
+			"descr": "Verzuckerungs-Rast",
 			"ramp": {
 				"rate": 1.0,
 				"temp": 72
-			}
-		},
-		{
-			"descr": "Halten Verzuckerungs-Rast",
+			},
 			"hold": {
-				"duration": 3000,
+				"duration": 30,
 				"stirrer": {
 					"speed": 35,
 					"interval_time": 120
@@ -94,19 +88,15 @@
 		},
 		{
 			"water_mass": 20,
-			"descr": "Heizen Abmaischen",
+			"descr": "Abmaischen",
+			"user_message": "Quittieren zum Abmaischen",
+			"user_wait_for_continue": true,
 			"ramp": {
 				"rate": 1.0,
 				"temp": 76
-			}
-		},
-		{
-			"water_mass": 20,
-			"descr": "Halten Abmaischen",
-			"user_message": "Quittieren zum Abmaischen",
-			"user_wait_for_continue": true,
+			},
 			"hold": {
-				"duration": 300,
+				"duration": 30,
 				"stirrer": {
 					"speed": 50
 				}