TST: add regression tests for affine

Test we're getting the same affine as we were (it may be the wrong
affine, but at least it is our affine).

Test we're getting the same-ish 3x3 component as the Philips generated
NIfTI image.
DOC: fix errors in top-level docstring

A few typos and thinkos
DOC: fix tiny typo in docstring
RF: refactor parrec affine, possibly breaks stuff

* Rephrase the transformations in terms of LPS instead of RAS in the
  hope of making the code easier to read
* Remove safety-net for the rotations, I'm confident the r2agui approach
  and euler2mat do the same thing.  For clarity, unpack the order of
  rotations with euler2mat in case we need to fix the order later.
* Use voxel FOV to get FOV offset in voxel space, then transform to get
  the FOV offset in mm

It's possible this refactoring breaks or fixes handling of
non-transverse images - we really need some of those to test against.