Refactor L0_loading_data.ipynb to use PyleoTUPS for data loading

[doswal]

The current notebook L0_loading_data.ipynb loads data using direct API calls.

To improve maintainability and align with current LinkedEarth tools, the data-loading section shall be refactored to use PyleoTUPS instead of raw API access.

Goals:

• Replace direct API calls with equivalent PyleoTUPS functions
• Preserve existing notebook functionality and outputs

[khider]

@doswal Should be L0_loading_data