Refactored _FileOrThing so:
 - it can be used on methods.
 - it keeps positional/keyword arguments separate
 - it can be applied to all arguments passed to a function

style. Fix to LOAD comparison - identity, not equality, otherwise ndarrays
will trigger an error.

affines

one instance, if two or more threads accessed a method at the same time.

state. Image.save ensures that file name has an extension. MGHImage uses both
of these changes so that its initial save state is accurate, and when it
is saved, it is saved as a nifti