public PepXmlReader(string filePath)
{
FilePath = filePath;
_document = new XmlDocument();
_document.Load(filePath);
_root = _document.DocumentElement;
_nsmgr = new XmlNamespaceManager(_document.NameTable);
_nsmgr.AddNamespace("pepxml", "http://regis-web.systembiology.net/pepXML");
}