Move Sud step's target temperature from ramp.temp to step.temperature

A step's target temperature applies to the whole step (it's what a
ramp ramps to and what a hold then holds at), not just its ramp phase
- promote it from a nested "ramp": {"temp": ...} to a step-level
"temperature" field, defaulted like descr/grain_mass/etc. Updates all
consumers (Sud._build_step, SudTask, the GUI's forecast estimator,
demo_sud.py) and migrates sude/sud_0010.json and the README.

sude/sud_0010.json

@@ -10,9 +10,9 @@
 			"user_wait_for_continue": false,
 			"grain_mass": 5.21,
 			"water_mass": 22,
+			"temperature": 0,
 			"ramp": {
 				"rate": 1.0,
-				"temp": 0,
 				"stirrer": {
 					"speed": 50,
 					"interval_time": 0,
@@ -32,18 +32,18 @@
 	"steps": [
 		{
 			"grain_mass": 0,
-			"descr": "Heizen f\u00fcr Einmaischen",
+			"descr": "Heizen für Einmaischen",
+			"temperature": 57,
 			"ramp": {
 				"rate": 1.8,
-				"temp": 57,
 				"stirrer": {
 					"speed": 75
 				}
 			}
 		},
 		{
-			"descr": "Einmaischen einf\u00fcllen",
-			"user_message": "Bitte Malz einf\u00fcllen und best\u00e4tigen",
+			"descr": "Einmaischen einfüllen",
+			"user_message": "Bitte Malz einfüllen und bestätigen",
 			"user_wait_for_continue": true,
 			"hold": {
 				"stirrer": {
@@ -59,9 +59,9 @@
 		},
 		{
 			"descr": "Glucose-Rast",
+			"temperature": 63,
 			"ramp": {
-				"rate": 1.0,
-				"temp": 63
+				"rate": 1.0
 			},
 			"hold": {
 				"duration": 40,
@@ -74,9 +74,9 @@
 		},
 		{
 			"descr": "Verzuckerungs-Rast",
+			"temperature": 72,
 			"ramp": {
-				"rate": 1.0,
-				"temp": 72
+				"rate": 1.0
 			},
 			"hold": {
 				"duration": 30,
@@ -91,9 +91,9 @@
 			"descr": "Abmaischen",
 			"user_message": "Quittieren zum Abmaischen",
 			"user_wait_for_continue": true,
+			"temperature": 76,
 			"ramp": {
-				"rate": 1.0,
-				"temp": 76
+				"rate": 1.0
 			},
 			"hold": {
 				"duration": 30,
@@ -103,4 +103,4 @@
 			}
 		}
 	]
-}
+}