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.

client/brewpi_gui.py

@@ -139,7 +139,7 @@ class SudForecastPlot(FigureCanvasQTAgg):
 			hold = step.get('hold')
 			if ramp is not None:
 				rate = ramp.get('rate', 0)
-				target = ramp['temp']
+				target = step['temperature']
 				if rate > 0:
 					t.append(t[-1] + abs(target - theta[-1]) / rate * 60.0)
 					theta.append(target)