feature/vtgnmea adds Vector Track and Speed over Ground (VTG) NMEA sentence to gdpsdlocatonagent

Added Vector Track and Speed over Ground (VTG) NMEA sentence support to
gdpsdlocatonagent. Velocity location events are used to generate VTG
sentences.

Submitted-by: Leonid Veytser 
Developed-by: James Kirsch

Patched submitted via email. Slight modifications made to better conform with module style.
```Diff
diff --git a/src/agents/gpsdlocation/agent.cc b/src/agents/gpsdlocation/agent.cc
index ef5f2b1..34d4e00 100644
--- a/src/agents/gpsdlocation/agent.cc
+++ b/src/agents/gpsdlocation/agent.cc
@@ -3,6 +3,7 @@
  * Copyright (c) 2008-2009 - DRS CenGen, LLC, Columbia, Maryland
  * All rights reserved.
  *
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
@@ -60,9 +61,12 @@ EMANE::Agents::GPSDLocation::Agent::Agent(NEMId nemId,
   masterPTY_{},
   slavePTY_{},
   bHaveInitialPosition_{false},
+  bHaveInitialVelocity_{false},
   dLatitudeDegrees_{},
   dLongitudeDegrees_{},
   dAltitudeMeters_{},
+  dAzimuthDegrees_{},
+  dMagnitudeMetersPerSecond_{},
   timerId_{}{}
 
 EMANE::Agents::GPSDLocation::Agent::~Agent(){}
@@ -286,6 +290,13 @@ void EMANE::Agents::GPSDLocation::Agent::processEvent(const EventId & eventId,
           dLatitudeDegrees_ = position.getLatitudeDegrees();
           dLongitudeDegrees_ = position.getLongitudeDegrees();
           dAltitudeMeters_ = position.getAltitudeMeters();
+          auto velocity = iter->getVelocity();
+	  if (velocity.second == true) // valid
+            {
+	      dAzimuthDegrees_ = velocity.first.getAzimuthDegrees();
+	      dMagnitudeMetersPerSecond_ = velocity.first.getMagnitudeMetersPerSecond();
+              bHaveInitialVelocity_ = true;
+            }
           bHaveInitialPosition_ = true;
         }
     }      
@@ -308,6 +319,18 @@ void  EMANE::Agents::GPSDLocation::Agent::processTimedEvent(TimerEventId,
                               dLatitudeDegrees_,
                               dLongitudeDegrees_,
                               dAltitudeMeters_);
+
+      if(bHaveInitialVelocity_)
+        {
+          sendSpoofedGPVTG(dAzimuthDegrees_, dMagnitudeMetersPerSecond_);
+
+          LOGGER_STANDARD_LOGGING(pPlatformService_->logService(),
+                                 DEBUG_LEVEL,
+                                 "gpsdlocationagent NEM: %hu azm: %lf mag: %lf",
+                                 nemId_,
+                                 dAzimuthDegrees_,
+                                 dMagnitudeMetersPerSecond_);
+        }
     }
 }
 
@@ -441,6 +464,28 @@ void  EMANE::Agents::GPSDLocation::Agent::sendSpoofedNMEA(double dLatitude, doub
     }
 }
 
+void  EMANE::Agents::GPSDLocation::Agent::sendSpoofedGPVTG(double dAzimuth, double dMagnitude)
+{
+  char buf[1024];
+
+  double dMagnitudeKnots = dMagnitude * 1.94384;
+  double dMagnitudeKilometerPerHour = (dMagnitude * 3600.0) / 1000.0;
+
+  /* NMEA GPVTG */
+  snprintf(buf,sizeof(buf),"$GPVTG,%.1f,%C,%.1f,%C,%.1f,%C,%.1f,%C",
+           dAzimuth,
+           'T', 
+           dAzimuth,
+           'M',
+           dMagnitudeKnots,
+           'N',
+           dMagnitudeKilometerPerHour,
+           'K'
+           );
+  
+  doCheckSumNMEA(buf,sizeof(buf));
+  write(masterPTY_,buf,strlen(buf));
+}
 
 void  EMANE::Agents::GPSDLocation::Agent::doCheckSumNMEA(char *buf, size_t len)
 {
diff --git a/src/agents/gpsdlocation/agent.h b/src/agents/gpsdlocation/agent.h
index 714086f..9ad4f55 100644
--- a/src/agents/gpsdlocation/agent.h
+++ b/src/agents/gpsdlocation/agent.h
@@ -92,9 +92,12 @@ namespace EMANE
         bool bGPSDConnectionEnabled_;
         ACE_FILE_IO pseudoTerminalNameFile_;
         bool bHaveInitialPosition_;
+        bool bHaveInitialVelocity_;
         double dLatitudeDegrees_;
         double dLongitudeDegrees_;
         double dAltitudeMeters_;
+	double dAzimuthDegrees_;
+	double dMagnitudeMetersPerSecond_; 
         TimerEventId timerId_;
   
         /**
@@ -106,6 +109,15 @@ namespace EMANE
          *
          */
         void sendSpoofedNMEA(double dLatitude, double dLongitude, double dAltitude);
+
+        /**
+         * Convert attitude in to NMEA strings and send to gpsd via pseudo  terminal
+         *
+         * @param dAzimuth   Azimuth in degrees
+         * @param dMagnitude Magnitude in meters per second
+         *
+         */
+	void sendSpoofedGPVTG(double dAzimuth, double dMagnitude);
       
         void doCheckSumNMEA(char *buf, size_t len);
       };
```