Temporarily disable the api.alturl test because the feature now requires a read-only dataset and OpenZGY doesn't make those yet.

Improved error messages. SDAPI exceptions are now mostly wrapped in a ZgyError exception, allowing OpenZGY to substitute a more readable message in the (regrettably few) cases it can do so.

Adjust the unit tests for reopen(), remove a few features that will not be supported in the not-faked version.

Pedantically correct handling of low resolution data at edges. This removes another impediment to storing 2d data in ZGY.

Remove some impediments to storing 2d data in ZGY. Not sure if that will ever be supported though. Added some very long comments.