Add Vehicle fact timeToHome

TimeToHome fact calculates the time in seconds till the UAV reaches the
home position. We take the distance to home and divide it by the
current ground speed to get the seconds till we arrive at the home
point. I did not add any fancy logic to find out if we actually fly
towards the home point or are in RTL so the value shows increasing
seconds even if we fly away from the home point

src/Vehicle/Vehicle.cc

@@ -91,6 +91,7 @@ const char* Vehicle::_altitudeTuningSetpointFactName = "altitudeTuningSetpoint";
 const char* Vehicle::_flightDistanceFactName =      "flightDistance";
 const char* Vehicle::_flightTimeFactName =          "flightTime";
 const char* Vehicle::_distanceToHomeFactName =      "distanceToHome";
+const char* Vehicle::_timeToHomeFactName =          "timeToHome";
 const char* Vehicle::_missionItemIndexFactName =    "missionItemIndex";
 const char* Vehicle::_headingToNextWPFactName =     "headingToNextWP";
 const char* Vehicle::_headingToHomeFactName =       "headingToHome";
@@ -154,6 +155,7 @@ Vehicle::Vehicle(LinkInterface*             link,
     , _flightDistanceFact           (0, _flightDistanceFactName,    FactMetaData::valueTypeDouble)
     , _flightTimeFact               (0, _flightTimeFactName,        FactMetaData::valueTypeElapsedTimeInSeconds)
     , _distanceToHomeFact           (0, _distanceToHomeFactName,    FactMetaData::valueTypeDouble)
+    , _timeToHomeFact               (0, _timeToHomeFactName,        FactMetaData::valueTypeDouble)
     , _missionItemIndexFact         (0, _missionItemIndexFactName,  FactMetaData::valueTypeUint16)
     , _headingToNextWPFact          (0, _headingToNextWPFactName,   FactMetaData::valueTypeDouble)
     , _headingToHomeFact            (0, _headingToHomeFactName,     FactMetaData::valueTypeDouble)
@@ -439,6 +441,7 @@ void Vehicle::_commonInit()
     _addFact(&_flightDistanceFact,      _flightDistanceFactName);
     _addFact(&_flightTimeFact,          _flightTimeFactName);
     _addFact(&_distanceToHomeFact,      _distanceToHomeFactName);
+    _addFact(&_timeToHomeFact,          _timeToHomeFactName);
     _addFact(&_missionItemIndexFact,    _missionItemIndexFactName);
     _addFact(&_headingToNextWPFact,     _headingToNextWPFactName);
     _addFact(&_headingToHomeFact,       _headingToHomeFactName);
@@ -1023,6 +1026,9 @@ void Vehicle::_handleVfrHud(mavlink_message_t& message)
         _altitudeTuningOffset = vfrHud.alt;
     }
     _altitudeTuningFact.setRawValue(vfrHud.alt - _altitudeTuningOffset);
+    if (!qIsNaN(vfrHud.groundspeed) && !qIsNaN(_distanceToHomeFact.cookedValue().toDouble())) {
+      _timeToHomeFact.setRawValue(_distanceToHomeFact.cookedValue().toDouble() / vfrHud.groundspeed);
+    }
 }
 
 void Vehicle::_handleNavControllerOutput(mavlink_message_t& message)

src/Vehicle/Vehicle.h

@@ -302,6 +302,7 @@ public:
     Q_PROPERTY(Fact* rangeFinderDist    READ rangeFinderDist    CONSTANT)
     Q_PROPERTY(Fact* flightDistance     READ flightDistance     CONSTANT)
     Q_PROPERTY(Fact* distanceToHome     READ distanceToHome     CONSTANT)
+    Q_PROPERTY(Fact* timeToHome         READ timeToHome         CONSTANT)
     Q_PROPERTY(Fact* missionItemIndex   READ missionItemIndex   CONSTANT)
     Q_PROPERTY(Fact* headingToNextWP    READ headingToNextWP    CONSTANT)
     Q_PROPERTY(Fact* headingToHome      READ headingToHome      CONSTANT)
@@ -679,6 +680,7 @@ public:
     Fact* rangeFinderDist                   () { return &_rangeFinderDistFact; }
     Fact* flightDistance                    () { return &_flightDistanceFact; }
     Fact* distanceToHome                    () { return &_distanceToHomeFact; }
+    Fact* timeToHome                        () { return &_timeToHomeFact; }
     Fact* missionItemIndex                  () { return &_missionItemIndexFact; }
     Fact* headingToNextWP                   () { return &_headingToNextWPFact; }
     Fact* headingToHome                     () { return &_headingToHomeFact; }
@@ -1358,6 +1360,7 @@ private:
     Fact _flightDistanceFact;
     Fact _flightTimeFact;
     Fact _distanceToHomeFact;
+    Fact _timeToHomeFact;
     Fact _missionItemIndexFact;
     Fact _headingToNextWPFact;
     Fact _headingToHomeFact;
@@ -1413,6 +1416,7 @@ private:
     static const char* _flightDistanceFactName;
     static const char* _flightTimeFactName;
     static const char* _distanceToHomeFactName;
+    static const char* _timeToHomeFactName;
     static const char* _missionItemIndexFactName;
     static const char* _headingToNextWPFactName;
     static const char* _headingToHomeFactName;

src/Vehicle/VehicleFact.json

@@ -95,6 +95,13 @@
     "units":            "m"
 },
 {
+    "name":             "timeToHome",
+    "shortDesc": "Time to Home",
+    "type":             "double",
+    "decimalPlaces":    1,
+    "units":            "s"
+},
+{
     "name":             "headingToHome",
     "shortDesc": "Heading to Home",
     "type":             "double",