check Zentra DB for existing data.

[Abel-Durham]

If the initial model be run more than one time, it will duplicate downloading values in the DB. Therefore, we need to add a function to double check it exists or not. Maybe something like:

SELECT MAX(date) from calculations_zentrareading;

to retrieve maximum value, e.g.

           max           
------------------------
 2022-09-19 23:55:00+00

We can then limit the query to the zentra API to only retrieve data after this date