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.

client/brewpi_gui.py

@@ -143,7 +143,7 @@ class SudForecastPlot(FigureCanvasQTAgg):
 					t.append(t[-1] + abs(target - theta[-1]) / rate * 60.0)
 					theta.append(target)
 			elif hold is not None:
-				t.append(t[-1] + hold.get('duration', 0))
+				t.append(t[-1] + hold.get('duration', 0) * 60.0)
 				theta.append(theta[-1])
 		return t, theta