Liquid-crystalline polyethers based on conformational isomerism. 16. Hexagonal columnar phase (.PHI.h) in a nondiscotic copolyether based on 1,2-bis(4-hydroxyphenyl)ethane, 1,8-dibromooctane, and 1,12-dibromododecane, and the novel 2-dimensional-3-dimensional .PHI.h-sB transition