[MRG] missing <bytes> cast causing random string problems

I was getting various warnings like this:

WARNING:py.warnings] RootNumpyUnconvertibleWarning: cannot convert leaf TkrNumStripsInLyr00 of branch TkrNumStripsInLyr00 with type P�� (skipping)
WARNING:py.warnings] RootNumpyUnconvertibleWarning: cannot convert leaf CalTrSizeTkrT99 of branch CalTrSizeTkrT99 with type � (skipping)

where it was obvious that the ``ltype`` string wasn't being interpreted correctly. I tracked down the problem to this missing ``<bytes>`` cast to interpret as a python string.