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.
Hmm.  There are some problems with making `nib.load('test.par').save('test.nii')` the same as the output for `parrec2nii`.

* I tend to think of the individual load routines as loading the data in the way that is most obvious for the image format.  In this case, that would be in the orientation the voxels are stored.  It's tough to explain why the format should load in LAS orientation rather than the orientation the image is saved in, where the answer is "FSL DTI routines seem to prefer data and bvecs in this orientation'.  Would we have to do this for every image type for which we can generate bvecs?
* If we do reorient the image, the header information and affine no longer corresponds to the voxel orientation in the image.  That could be confusing.