jacobian is an output image in convertwarp

passed as a sequence of CL arguments, not a single value

This enables a common use case - logging the command but not the standard output

This allows @fileOrArray to be more liberal without returning a lot of
key : None in the results.

It's often helpful to be able to leave an argument as None and only set it
if you want to use it.

Previously this would assume $FSLDEVDIR/bin whenever $FSLDEVDIR was set
even if there was no matching command there, but there was one in $FSLDIR

If specified, single-character arguments always take single dash prefix even if
argument style is --. This is a standard variation.

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.