Add script to display balancing settlement system prices

[14Richa]

Description

This pull request introduces a new script (system_price.py) demonstrating the retrieval and display of balancing settlement system prices from the Elexon API.

Output:

Below is the screenshot of the output.

Fixes #27

Checklist:

• [X] My code follows OCF's coding style guidelines
• [X] I have performed a self-review of my code.
• [X] I have checked my code and corrected any misspellings