[dhtmlxgantt] restore calculateDuration/calculateEndDate/isWorkTime signatures
[dhtmlxgantt] restore calculateDuration/calculateEndDate/isWorkTime signatures

georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,dsebastien/DefinitelyTyped,markogresak/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,magny/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,magny/DefinitelyTyped,borisyankov/DefinitelyTyped