It cost pounds 135m, it took longer to finish than the Taj Mahal: there were rumours that the millennium would have to be postponed for its completion. But Tate Bankside will open its massive doors in May 2000, and we will finally be able to see what Swiss architects Herzog and de Meuron have been up to inside the power station. Or, rather, Tate Bankside will not open its massive doors. The gallery has been re-dubbed Tate Modern, presumably in the belief that mere geography would not be enough to separate it in the art lover's mind from the other Tate across the river. (This, just to cross all t's and dot i's, will be re-launched as Tate Britain in March.) On the whole, the …