Add version GPX

[otonielloliveira]

Problem description

1. Library version or last commit you have
2. PHP version you use
3. Detailed problem description

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Code snippet you use

class phpGPX

$gpx->version = isset($xml['version']) ? (string)$xml['version'] : null;

class GpxFile 

	/**
	 * Version about the file.
	 * The original GPX 1.1 attribute.
	 * @var Version|null
	 */
	public $version;
	
	$this->version = null;
	
	'version' => SerializationHelper::stringOrNull($this->version),