The current intepreter is in sys.executable
Having the option to use a different intepreter seems like asking for
trouble (as an older interpreter might not be able to read the pickle
files)

If a logdir has been set that directory should be provided rather than the directory from which the job was submitted (which is what the logdir defaults to).

will probably change.

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