Fix inconsistency between OutputToNRAR and OutputToH5

Added a helper function __AddFileNamePrefix, which performs the same
string manipulations in both OutputTo{H5,NRAR}. Each was missing
a critical piece of logic:

  * OutputToH5 mishandled the case when the FileName did not contain
    a directory component.

  * OutputToNRAR did not include the FrameType in the FileName.
OutputToNRAR: fix an error message typo
Extensions.py: Add OutputTo{H5,NRAR} to PNWaveform interface