Cellarr

* Implement dunder methods `__len__`, `__repr__` and `__str__` for the `CellArrDatasetSlice` class
* Add property `shape` to the same class
* Improve package load time

- Thanks to [tony-kuo](https://github.com/tony-kuo), the package now includes a built-in dataloader for the pytorch-lightning framework,
for single cells expression profiles, training labels, and study labels. The dataloader uniformly samples across training labels and study labels to create a diverse batch of cells.

- Minor fixes for CSV to TileDB conversion for the `cell_metadata` object.

This is the first release of the package to support both creation and access to large
collection of files based on TileDB.

- Provide a build method to create the TileDB collection from a series of data objects.
- Provides `CellArrDataset` class to query these objects on disk.
- Implements access and coerce methods to interop with other experimental data packages.