A typo had slipped through the last commit for Array4D. The Corripio slope computation was till wrong, this has finally been fixed and checked. The slope default algorithm selection was not kept through BufferedIOHandler, this has been fixed. Moreover, unecessary copies of grids were being made in BufferedIOHandler and have been removed. The tests that were not passing anymore because of the slope changes have been updated, including improved error reporting in dem_reading.