Chunk-aligned indexing

[TomNicholas]

Allows you to subset ManifestArray objects, as long as your indexer slice aligns with chunk boundaries.

• [x] Closes #51
• [x] Tests added
• [x] Tests passing
• [x] Full type hint coverage
• [ ] Changes are documented in docs/releases.rst
• [ ] New functions/methods are listed in api.rst
• [ ] New functionality has documentation