Redo data setting in simulation

[anth-volk]

Fixes #154
Fixes PolicyEngine/policyengine-api-v2#212

What Changed

• Renamed constants.py to /data/datasets.py
• Added a dict mapping datasets that have a required time_period value to said value
• Corrected Google Cloud Storage links
• Refactored _set_data to handle properly pass dataset addresses to -core's Dataset.from_file function in order to enable downloading
• Extricated time_period-setting code from data-setting code
• Improved _set_data code quality and better separated concerns
• Implemented a consistent variable naming structure around filepaths: filename = unique file name + extension, file_address = URI [+ bucket name] + filename
• Added tests

Why This Change

• Enables GCS datasets to download
• Reduces confusion around filepath passage
• Improves maintainability and clarity of _set_data function
• Improves test coverage of the same function

Technical Details

N/A

[anth-volk]

Now ready for review