OpenZGY update support

Support updating existing ZGY files, limited to uncompressed on-prem. Finalizing a file will rebuild all the low resolution data every time. This may be a performance problem.

Applications can call close_incomplete() instead of close() if it is known that the file will be re-opened later to add more data. This sidesteps the performance issue. The incomplete file is still usable but will only contain full resolution data (will have nlods==1) and will report empty histogram and statistcs.