Update version number for ZFP compressed files from 3 to 4.
This change is almost a no-brainer that should have been made as soon as ZFP support was added. I failed to realize it at that point. Fortunately it isn't much of a problem that the change was delayed until now.
The only issue is that if you use OpenZGY to write a ZFP-compressed file after this change then that file will not be readable by older versions of OpenZGY. The other direction works fine.