DOC: reformat affine docstring

Line-wrapping
NF: add routine to make output space bounding box

``vox2out_vox`` routine gives the affine, shape to resample to, to give
an image oriented to its output space.  This can be useful for checking
image orientation.
NF: function to make affine for selecting slice

Affine to express selection of slice from volume.
DOC: docstring and comment changes

Suggestions from Yarick on PR
RF: change to mapped_voxels as vox2out_vox input

Also refactor tests to be a little less repetive.
Merge branch 'to-output-space' into main-master

* to-output-space:
  RF: change to mapped_voxels as vox2out_vox input
  DOC: docstring and comment changes
  NF: function to make affine for selecting slice
  NF: add routine to make output space bounding box
  DOC: reformat affine docstring