ENH: fast reads on large gzip files

Fixes #209
BUG: fix previous commit

Acutally use the opening logic
RF: put gzip chunk constant into module global

Allows user to set max chunk size.
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.
DOC: zooms then rotations in tutorial

Follow Alexis' suggestion of having rotations after zooms in the section
on combining transformations.
MAINT: add .gitignore to scripts directory

To ignore output of scripts.
DOC: point to image generation script

in comment in RST file.  Add more specifics of needed images in script
comments.
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
RF+DOC: refactor Analyze-type header conversion

Refactor header conversion routine to use mapping if available, then
basic conversion always.

Document the ``as_analyze_map`` interface in docstring.

Add ``as_analyze_map`` tests and as API documentation.

Rename ``_set_format_specifics`` to ``_clean_after_mapping`` to make
clear that the routine is to set needed defaults after creating a header
from a mapping.

Remove unused ``_set_format_specifics`` from freesurfer/mghformat
RF: move script test machinery into own module

Use scriptrunner module that started off in the delocate project. Well,
it started in the test_scripts.py module here, but I refactored it in
the delocate project, and the refactoring is coming back home.
BF: workaround missing __package__ in Python 3.2

Python 3.2 does not set __package__; work around for now.
RF: change "below" to "above" for higher directory

I tend to think of directories nearer root as being 'down', it looks
more common to think of this as being 'up', change according to Ariel's
suggestion.
DOC: fixing docstrings for images

Fix some old and incorrect docstrings.

Update an error message with newer string formatting.
Merge branch 'use-scriptrunner' into main-master

* use-scriptrunner:
  RF: change "below" to "above" for higher directory
  BF: workaround missing __package__ in Python 3.2
  RF: move script test machinery into own module
Merge branch 'refactor-analyze-map' into main-master

* refactor-analyze-map:
  RF+DOC: refactor Analyze-type header conversion
Merge pull request #1 from matthew-brett/module-global-gzip

MRG: add module global to gzip as discussed