RF: specify endiannes of PARREC data

Is PARREC data always little-endian?  Guess that it is for now.
RF: specify endianness on nifti write

PARREC to nifti should specify data endianness.
Merge pull request #272 from matthew-brett/parrec-endianness

MRG: specify endianness of PARREC data

Guess that REC data is always little-endian.