Improved process for the chlorination of 3a, 4, 7, 7a - tetrahydro - 4, 5, 6, 7, 8, 8 hexachloro- - 4, 7 - methanoindene, chlorinated product obtained by virtue of this process and insecticide comprising the 3a, 4, 7, 7a - tetrahydro - 4, 5, 6, 7, 8, 8 - hexachloro- - 4, 7 - methanoindene chlorinated