COLMAR , a fifty-minute train ride south of Strasbourg, has sprawled unattractively on both sides of the train tracks, but the old centre remains typically and whimsically Alsatian, with crooked houses, half-timbered and painted, on crooked lanes - all extremely pretty and very touristy. Colmar's attractions don't stop at its buildings; it is also the proud possessor of one of the last and most extraordinary of all Gothic paintings - the altarpiece for St Anthony's monastery at Issenheim, painted by Mathias Grünewald.