This PR closes #357 by cleaning up the API of DataFolder and extending the tests to cover all functionality.

The main changes in the PR are:

• Combining functionality for handling metadata tables and normal tables
• Adding a new write method that can be used for both normal tables and time series tables
• Cleaning up the public API of DataFolder
• Reordering methods and tests to better match the rest of the repository
• Adding tests for all public methods in DataFolder
• Adding tests for all public methods in DeltaTableWriter