Include GPS accuracy in GPX export

Add a new "accuracy" tag to the opentracks extension as there was no
other extension providing this tag that could be found.

Signed-off-by: Brian J. Murrell <brian@interlinx.bc.ca>

doc/opentracks-schema-1.0.xsd

@@ -29,4 +29,13 @@
             </xsd:documentation>
         </xsd:annotation>
     </xsd:element>
+    <xsd:element name="accuracy"
+        type="xsd:float">
+        <xsd:annotation>
+            <xsd:documentation xml:lang="en">
+                Accuracy in meters of the current TrackPoint.
+                Only used in GPX.
+            </xsd:documentation>
+        </xsd:annotation>
+    </xsd:element>
 </xsd:schema>

src/main/java/de/dennisguse/opentracks/io/file/exporter/GPXTrackExporter.java

@@ -358,6 +358,10 @@ public class GPXTrackExporter implements TrackExporter {
                     trackPointExtensionContent += ("<opentracks:loss>" + ALTITUDE_FORMAT.format(cumulativeLoss) + "</opentracks:loss>\n");
                 }
 
+                if (trackPoint.hasHorizontalAccuracy()) {
+                    trackPointExtensionContent += ("<opentracks:accuracy>" + DISTANCE_FORMAT.format(trackPoint.getHorizontalAccuracy().toM()) + "</opentracks:accuracy>");
+                }
+
                 cumulativeDistance = Distance.ofOrNull(cumulateSensorData(trackPoint, sensorPoints, (tp) -> tp.hasSensorDistance() ? tp.getSensorDistance().toM() : null));
                 if (cumulativeDistance != null) {
                     trackPointExtensionContent += ("<opentracks:distance>" + DISTANCE_FORMAT.format(cumulativeDistance.toM()) + "</opentracks:distance>\n");