WIP: refactoring parrec2nii while adding tests

Refactoring parrec2nii to make it easier to save in canonical array
orientation.  First add tests for minmax and scaling options to make
sure I don't break anything.
I'm happy with the second option; use the hack(ish) command line solution
for now, with a note -- or better, open issue -- reminding us to work out a
more systematic solution later. That way the pretty comprehensive release
doesn't have to (continue to) be blocked by a minor corner case annoyance
with a usable workaround :)