Create GPX object from XML-string

[jenetics]

Add factory methods which allows the creation of GPX objects from a GPX-XML string.

final String gps = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
    "<gpx version=\"1.1\" creator=\"JPX - Java GPX library\" xmlns=\"http://www.topografix.com/GPX/1/1\">\n" +
    "    <trk>\n" +
    "        <trkseg>\n" +
    "            <trkpt lat=\"48.2081743\" lon=\"16.3738189\">\n" +
    "                <ele>160.0</ele>\n" +
    "            </trkpt>\n" +
    "            <trkpt lat=\"48.2081743\" lon=\"16.3738189\">\n" +
    "                <ele>161.0</ele>\n" +
    "            </trkpt>\n" +
    "            <trkpt lat=\"48.2081743\" lon=\"16.3738189\">\n" +
    "                <ele>162.0</ele>\n" +
    "            </trkpt>\n" +
    "        </trkseg>\n" +
    "    </trk>\n" +
    "</gpx>";

final GPX gpx = GPX.fromString(gps);

[jenetics]

Was part of #25.

GPX.reader().fromString(...);