Add `data_preparation/generate_customers.py`, a script that takes the
`base_data.json` file generated by `get_base_data.py` and randomly
samples a given number of customers.
To simplify things, each customer is assigned exactly one gas and one
electricity meter and each of them is read between 1 and 10 times.
The full data including meters, meter readings and dates as well as
customers and addresses is stored in a final JSON file named
`customers.json`.
