Planner: correctly deal with units in Ascent/descent rates

This adds two changes
a) it uses rint() to make sure we don't truncate the displayed values
b) it moves the update of the displayed values into a helper function that
   is also called whenever the settings change

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/diveplanner.h

@@ -169,6 +169,7 @@ slots:
 
 private:
 	Ui::plannerSettingsWidget ui;
+	void updateUnitsUI();
 };
 
 QString dpGasToStr(const divedatapoint &p);