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.
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
FIX: Support XYTZ order and multiple echos in PAR/REC files
ENH: Support for partial recordings, and overwrite checking
WIP: Add bvals/bvecs
FIX: Working for variable scale factors
ENH: Add dwell time
FIX: Verbose dwell output
FIX: Fix dwell time
FIX: Fix numpy
FIX: Move most work to parrec
FIX: Fix for truncated data
FIX: Fix tests
FIX: Fix openers