Intern

- **Improvements**
- Removes the `mock` library (thanks musicinmybrain!)
- Confirms that incoming upload data are C-contiguous prior to upload
- Extracts the `normalize_key` function to make slicing into convenience arrays more robust and reports voxel size relative to the requested resolution (thanks CBroz1!)

- **Features**
- A new convenience function `array.from_images` uploads an entire directory as a new BossDB dataset

- **Fixes**
- Resolves a TypeError when warning in the non-cloudvolume case.

- **Fixes**
- Resolves an issue with cloudvolume imports on non-cloudvolume-dependent resources.

- **Fixes**
- Resolves an issue with accessing cloudvolume-backed public data in BossDB with ZYX/XYZ axis-ordering

- **Convenience API**
- Adds support for channel-level metadata using the array().metadata dict-like API (87)
- Adds a `array().downsampled` property on the convenience API (84)
- Fixes voxel_unit getters and setters to be more consistent with convenience API constructor (83)
- Adds data-source inference to redirect requests to the correct `Remote` (91)
- Adds a metadata interface `Metadata()` for general metadata read/write access (92)
- **Improvements**
- Adds a `BossResource.public` boolean flag to mirror if a resource is publicly accessible (89)
- **Fixes**
- Closes the multiprocessing Pool to avoid too many open files when parallelizing requests (86)