Iterate over `settings.unit`, executing a `pickUnit` function for each item.
function pickUnits(dest, settings) {
        var units = settings.units;
        if (!isArray(units)) {
            error("Invalid units array in pick settings");
        }
        if (!(settings.namespace instanceof Gumup)) {
            error("Invalid namespace in pick settings");
        }
        var picked = {},
            srcDecls = settings.namespace._declarations;
        for (var i = 0, len = units.length; i < len; i++) {
            var reqName = units[i];
            if (!checkRequireName(reqName)) {
                error("Invalid unit name '" + reqName + "' in pick settings");
            }
            forEach(srcDecls, reqName, function(depName) {
                pickUnit(srcDecls, dest._declarations, depName, picked, {});
            });
        }
    }