Revert Sud hold "duration" back to minutes

Reverts 19dc7a9 ("Fix Sud hold duration units: seconds, not minutes")
per explicit instruction - duration is minutes again. Re-adds the
*60.0 conversion in Sud._advance() (hold_remaining is ticked in
simulated seconds), the matching conversion in the forecast plot's
_estimate_course(), and flips the README wording back.

components/sud.py

@@ -178,7 +178,9 @@ class Sud(AttributeChange):
 		if 'ramp' in next_step:
 			self.state = SudState.RAMPING
 		else:
-			self.hold_remaining = next_step['hold'].get('duration', 0)
+			# "duration" is in minutes; hold_remaining (ticked by tick()'s
+			# simulated-seconds dt) is in seconds.
+			self.hold_remaining = next_step['hold'].get('duration', 0) * 60.0
 			self.state = SudState.HOLDING
 
 		self.user_message = next_step.get('user_message')