Added cycling speed sensor.

Fixed speed and cadence computation.
Fixed that a sensor change in settings resulted in a disconnect for all connected sensors.

src/main/java/de/dennisguse/opentracks/util/UnitConversions.java

@@ -46,6 +46,8 @@ public class UnitConversions {
     // multiplication factor to convert kilometers to miles
     public static final double KM_TO_MI = 0.621371192;
 
+    public static final double MM_TO_M = 0.001;
+
     // Distance
     // multiplication factor to convert miles to feet
     public static final double MI_TO_FT = 5280.0;