Allow multiple filenames per partition when separating by metadata

[ayushdg]

Description

Closes #89

Adds an option to handle scenarios where there are multiple filenames in a single partition when writing with filename. This is typically true when files are read in with files_per_partition > 1.

Usage

dataset = DocumentDataset.read_json(path, files_per_partition=5, include_filename=True)
write_to_disk(dataset.df, output_file_dir=path, write_to_filename=True)

Checklist

• [X] I am familiar with the Contributing Guide.
• [x] New or Existing tests cover these changes.
• [X] The documentation is up to date with these changes.