Stavanger flourished in the early 19th century as a centre for the fishing trade and while other towns and cities in Norway have suffered with the decline of the industry, Stavanger has managed to keep its economy booming by diversifying, first into ship building and now into oil.   It is a city of two halves, a bustling and modern area of high-rise buildings and an historic old centre of warehouses, stock rooms, cobbled streets and clapboard residential buildings dating back to the 18th and 19th centuries.   The Cathedral, which dates from 1125, is an impressive building and the only cathedral in Norway to retain its original features.