By Michelle Nichols UNITED NATIONS (Reuters) -U.N. aid chief Martin Griffiths said on Thursday he was hopeful that the first shipment of grain from a Ukrainian Black Sea port could take place as early as Friday, but “crucial” details for the safe passage of vessels were still being worked out. Griffiths said Turkish, Russian and Ukrainian military officials were working with a U.N. team at a Joint Coordination Center in Istanbul to hammer out standard operating procedures for the deal agreed by the four parties last Friday. “That is a detailed negotiation based on the agreement,” Griffiths tol…