Pyzgy

- Enable negative UTM coordinates.
- Faster file opening
- Testing on MacOS

- Fix unequal il/xl increments

- Add `pyzgy.tools.dt` function to mirror segyio
- Add `SeismicReader.close()` function for use outside context manager
- Share filehandle between accessors for faster initialization
- Add SourceGroupScalar default value

- Use warnings rather than print

Rename package to pyZGY

Correct dependencies in setup.py