/// <summary>Save the mappings to a file</summary>
/// <param name='mapping'>the mapping object to store</param>
/// <param name='filename'>the name of the file</param>
public static void SaveMapping(this IMapping mapping, string filename)
{
using (var writer = new StreamWriter(filename))
foreach (int internal_id in mapping.InternalIDs)
{
writer.WriteLine("{0}\t{1}", internal_id, mapping.ToOriginalID(internal_id));
}
}