_totalProperties = function(body) {
        // https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=st&chap_sec=07.2&page=theory
        // http://output.to/sideway/default.asp?qno=121100087

        var properties = {
            mass: 0,
            area: 0,
            inertia: 0,
            centre: { x: 0, y: 0 }
        };

        // sum the properties of all compound parts of the parent body
        for (var i = body.parts.length === 1 ? 0 : 1; i < body.parts.length; i++) {
            var part = body.parts[i];
            properties.mass += part.mass;
            properties.area += part.area;
            properties.inertia += part.inertia;
            properties.centre = Vector.add(properties.centre, 
                                           Vector.mult(part.position, part.mass !== Infinity ? part.mass : 1));
        }

        properties.centre = Vector.div(properties.centre, 
                                       properties.mass !== Infinity ? properties.mass : body.parts.length);

        return properties;
    }
Returns the sums of the properties of all compound parts of the parent body.
@method _totalProperties
@private
@param {body} body
@return {}