Develop standalone example scripts for PLEXOS model conversion

[measrainsey]

Create individual example scripts that demonstrate end-to-end conversion of specific PLEXOS models.

Requirements:
Each script should:

• Look for PLEXOS model in expected directory structure
• Download model if not present locally
• Convert model to PyPSA network
• Run as standalone script with minimal setup

Implementation:

• Search for PLEXOS model under examples/data/
• If model not found → download it automatically
• Expected format: root/examples/data/[model_name]/
• Output: converted PyPSA network