env option

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

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

stdout/stderr streams go.

gives $FSLDEVDIR precedence, and also allows a global override.

stdout/stderr, and also redirects it to real stdout/stderr.

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.