testing. Tweaks to various wrapper functions. fslmaths returns results dict,
rather than file name (for testing).

be standardised. Subsequent refactorings to _FileOrThing class, to make
sure that wrapper functions can be updated correctly. runfsl function
passes kwargs through to run.

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.