use double precision when computing the mean

The datatype of the mean is also used to compute the sum. With sufficiently large images, this calculation was not correct.
fixed update for images larger than 16777216 pixels

large images get chunked. given a generic or affine camera, the proper offsets were not being applied for the chunks
check return codes from processes and forward on
rational to generic camera convertion special case fix

removed +1 because (rarely) the footprint of a level in the image pyramid would cover a region beyond the valid region of the RPC, causing the ray to fail to backproject with sufficient accuracy
CMake requires a CMakeLists.txt file in every folder