prefixed-outputs. Bet wrapper uses new outprefix option.

used as a wildcard - all files which begin with outprefix can be specified
as keyword arguments and LOADed.

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.