Refactor the `data.__init__.py` module

[jrycw]

Hello team,

This PR aims to address the Pandas dependency in reading datasets by introducing a unified Dataset API. The proposed approach allows users to retrieve datasets in a user-specified dataframe format. For example:

• To get a Pandas dataframe for the sza dataset, use Dataset.sza.to_pandas().
• To get a Polars dataframe for the same dataset, use Dataset.sza.to_polars().
• To get a PyArrow dataframe for the same dataset, use Dataset.sza.to_pyarrow() (implementation of _convert_to_pyarrow() and to_pyarrow() is needed to support this).

This way, users can use autocomplete to select both the dataset and the desired dataframe type.

To facilitate the transition:

• Each dataset name begins with an uppercase letter (e.g., Sza).
• A lowercase variable (e.g., sza) is provided as a Pandas dataframe, created using to_pandas().

If we decide to completely remove Pandas as a dependency in the future, the following tasks will be required:

• Remove the warning message.
• Remove the lowercase variables representing Pandas dataframes, and rename the dataset classes to lowercase.
• Update __all__.
• Update documentation and tests to reflect the new API.
• Address code sections marked with # remove pandas for further cleanup.

I’m confident there are other excellent approaches to tackle this issue, so please feel free to modify or reject this PR as needed.