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.
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
Merge pull request #210 from GaelVaroquaux/fast_gzip_read

MRG: fast reads on large gzip files

Fixes #209