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 think this process should be in the PARREC load routines instead of `parrec2nii`... I like to think of `nib.load('test.par').save('test.nii')` as being equivalent to `parrec2nii`, and putting this functionality in `parrec2nii` breaks this relationship. Or at least there should be a kwarg in `nib.load` like `reorient_LAS=True` (be default) that gets us the same behavior by fixing the image, affine, and bvecs as necessary.