Merge internal PR 525649, 534872, 537022: Heuristic for lowres data present

Fix the heuristic that decides when low resolution data is present.

One of the new features in OpenZGY is the ability to create a ZGY file with no resolution data. Application code will see nlods() returning 1.

The change was done without breaking the layout of ZGY files. Instead of storing some usable_nlods, the reader uses a heuristic to decide whether low resolution bricks are present. That heuristic turned out to be a lot more complex than expected. Especially when factoring in corner cases such as a constant-value survey (doesn't need finalizing for the lowres to be valid) or single-brick surveys (no lowres needed, so finalize does nothing).