RF: refactor parrec2nii image writing

Use new explicit scaling to save the image using the standard nibabel
mechanism.
BF+TST: fix / test saving of PAR into NIfTI header

We weren't saving the PAR header when asked to; fix and test.
WIP+BRK: apply RAS reorientation to image

Reorient data to RAS during conversion.

This breaks the tests that expect the data to be in the same orientation
for the PAR format as the converted NIfTI format.