Seismic-zfp

* Adds support for negative samples.

- Store z-unit scaled up by factor 1000
- Enabling μs/mm resolution
- SgzReader.zslice/f.samples returned as numpy float array
- Pull in latest from zgy2sgz:
- CDP_X, CDP_Y & SourceGroupScalar trace headers populated
- Fix integer overflow in calculating SGZ headers for large files

- Fix bug where trace headers were only accessible via segyio emulation
- Enable choice between reading trace headers individually or en-masse

- add basic CLI
- make zgy2sgz dependency optional
- add chunk_cache_size passthrough
- compress decimated unstructured segy files.