By Tassilo Hummel and Ardee NAPOLITANO PARIS (Reuters) -Traffic on the cross-Channel Eurotunnel will gradually resume following an agreement with unions, Eurotunnel said, after a strike by French workers interrupted traffic on the undersea link between Britain and continental Europe most of Thursday, upending Christmas holiday plans for thousand of travellers. Eurotunnel said in a statement that service on the LeShuttle transport service for motor vehicles would resume gradually from Thursday night and on the Eurostar passenger trains from Friday. Freight trains would also resume traffic from …