Add utility functionality

What's Changed

• Enable adding sub-directory sources of a zarr-file by @tibuch in #72
• Rename source by @constantinpape in #75
• Remove source by @constantinpape in #76
• Initialize segmentation table by @constantinpape in #77
• Add support for channel axis in source by @constantinpape in #73

New Contributors

• @tibuch made their first contribution in #72

Full Changelog: 0.3.0...0.3.1