ENH: convert option can convert files containing multiple transforms

Taking full advantage of the new implementation of ITKTransformFileReaderTemplate and ITKTransformFileWriterTemplate. The type of the input transform does not matter, and does not have to be specified. New flags have been therefore added ('2f' and '2d') but old flags ('d2f' and f2d') have been kept for backward compatibility purposes. Any transform type should also be supported now.