Allow application to retrieve compression factor and similar statistics.

Let application code retrieve the achieved compression ratio and the file version. Both are only meant to display as information to the user.

Case in point: Application code should not change its behavior based on the file version number. Because OpenZGY is supposed to hide that information. If special handling is needed then a more specific method will be provided. E.g. you might use stats->fileHasXYZCapability() instead of stats->version() >= 42.

The same rule applies to statistics such the number of empty, compressed, and regular bricks.