DataFlash: switched to float for GPS ground speed

same size and more precision

libraries/DataFlash/LogFile.cpp

@@ -721,7 +721,7 @@ void DataFlash_Class::Log_Write_GPS(const AP_GPS &gps, uint8_t i)
         latitude      : loc.lat,
         longitude     : loc.lng,
         altitude      : loc.alt,
-        ground_speed  : (uint32_t)(gps.ground_speed(i) * 100),
+        ground_speed  : gps.ground_speed(i),
         ground_course : gps.ground_course_cd(i),
         vel_z         : gps.velocity(i).z,
         used          : (uint8_t)(gps.primary_sensor() == i)

libraries/DataFlash/LogStructure.h

@@ -52,7 +52,7 @@ struct PACKED log_GPS {
     int32_t  latitude;
     int32_t  longitude;
     int32_t  altitude;
-    uint32_t ground_speed;
+    float    ground_speed;
     int32_t  ground_course;
     float    vel_z;
     uint8_t  used;
@@ -714,9 +714,9 @@ Format characters in the format string for binary log messages
     { LOG_PARAMETER_MSG, sizeof(log_Parameter), \
       "PARM", "QNf",        "TimeUS,Name,Value" },    \
     { LOG_GPS_MSG, sizeof(log_GPS), \
-      "GPS",  "QBIHBcLLeEefB", "TimeUS,Status,GMS,GWk,NSats,HDop,Lat,Lng,Alt,Spd,GCrs,VZ,U" }, \
+      "GPS",  "QBIHBcLLefefB", "TimeUS,Status,GMS,GWk,NSats,HDop,Lat,Lng,Alt,Spd,GCrs,VZ,U" }, \
     { LOG_GPS2_MSG, sizeof(log_GPS), \
-      "GPS2", "QBIHBcLLeEefB", "TimeUS,Status,GMS,GWk,NSats,HDop,Lat,Lng,Alt,Spd,GCrs,VZ,U" }, \
+      "GPS2", "QBIHBcLLefefB", "TimeUS,Status,GMS,GWk,NSats,HDop,Lat,Lng,Alt,Spd,GCrs,VZ,U" }, \
     { LOG_GPA_MSG,  sizeof(log_GPA), \
       "GPA",  "QCCCC", "TimeUS,VDop,HAcc,VAcc,SAcc" }, \
     { LOG_GPA2_MSG, sizeof(log_GPA), \