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 prefer synod, but oh well.

I did think about a function or method, but if this would return a parrec image, then we run into the same problem that we have to think about what would happen when the affine and file orientation is divorced from the header.   For example, we would have to decide whether we were going to modify the bvecs in the header, the scalings (need to be reshaped, can't any longer rely on shape (1, 1, z, t), the rotations etc in the affine (probably not).

We could instead return a nifti image by default, and for these we can create headers easily. Then we can't represent the bvals, bvecs, at least at the moment.

Options are to 

* punt on this problem for the release, converted images will look odd in FSL;
* use my hack for parrec2nii for now for the release, plan something better later;
* make a function / method to reorient the parrec files and try and work out what we need to keep track of for the header when copying.