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.

components/sud.py

@@ -157,7 +157,18 @@ class Sud(AttributeChange):
 			self._advance()
 
 	def temp_reached(self):
-		if self.state == SudState.RAMPING:
+		if self.state != SudState.RAMPING:
+			return
+		if 'hold' in self.step:
+			# Same step, ramp phase done - move into its hold phase rather
+			# than finishing the step. Re-assign self.step (AttributeChange
+			# fires on every assignment, not just on change) to re-trigger
+			# the step-changed callback so e.g. the hold's stirrer settings
+			# get (re-)applied.
+			self.hold_remaining = self.step['hold'].get('duration', 0) * 60.0
+			self.state = SudState.HOLDING
+			self.step = self.step
+		else:
 			self._finish_step()
 
 	def tick(self, dt):