No person shall be held guilty of an offence on account of any act or omission which did not, at the time of such act or omission, constitute such an offence and no penalty shall be imposed for any offence more severe than the penalty in force at the time such offence was committed. Nothing in this Article shall prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognized by the community of nations. It shall not be contravention of this Article to require the imposition of a minimum penalty for an offence provided that such penalty does not exceed the maximum penalty prescribed for such offence at the time such offence was committed.