Add `demand.py` module for Elexon API Data Retrieval

[14Richa]

This PR introduces a new Demand module to get demand data from the Elexon API.

Below is the list of the endpoints covered by this module:

1. Actual Total Demand: https://developer.data.elexon.co.uk/api-details#api=prod-insol-insights-api&operation=get-demand-actual-total-from-from-to-to
2. Indicative Peak Demand: https://developer.data.elexon.co.uk/api-details#api=prod-insol-insights-api&operation=get-demand-peak-indicative-data-data
3. Initial National Demand Outturn : Endpoint: /demand/outturn, /demand/outturn/stream, /demand/outturn/daily, /demand/outturn/daily/stream.
4. Peak Demand per Day (ITSDO): https://developer.data.elexon.co.uk/api-details#api=prod-insol-insights-api&operation=get-demand-peak
5. Triad Demand Peaks: https://developer.data.elexon.co.uk/api-details#api=prod-insol-insights-api&operation=get-demand-peak-triad-data-data

Fixes #15

Checklist:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.58%. Comparing base (8da3b1d) to head (5e28cc8). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16 +/- ## ======================================= Coverage 77.58% 77.58% ======================================= Files 7 7 Lines 58 58 ======================================= Hits 45 45 Misses 13 13 ```

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[peterdudfield]

Not needed now, as code was auto generated