convert_to_irreversible preserves subsystem
convert_to_irreversible preserves subsystem

Fix reported by Douglas McCloskey on 2/8/14 but somehow never got
committed.

zakandrewking/cobrapy,jeicher/cobrapy,aebrahim/cobrapy,zakandrewking/cobrapy,JuBra/cobrapy,aebrahim/cobrapy,jeicher/cobrapy,JuBra/cobrapy