0. AtlasRegistry is no longer thread-safe. It was causing too much
   trouble, and loading atlas descriptions is not a bottle neck anyway,
   so it's completely unnecessary.

1. AtlasDescriptions are now stored as a sorted list.

2. All atlases (including those in $FSLDIR) are saved.

3. More AtlasRegistry methods to rescan, check, and remove atlases.

Little bugfix in Notifier - calling skip/enabled methods with no listeners
registered would result in error.

dimensionality

changed data.

identifying reference image associated with a FIRST vtk file.

space as another.

and hashbang into every .py file.

path looks like part of an incomplete file group.

when called programmatically.

programmatically invoked.

the default file extensions (i.e. the FSLOUTPUTTYPE) - this is used by
the fslpy_immv/imcp command line scripts.

appropriate extension

covered 'at least two elements' anyway. Maybe this will bite me, I
don't know.