[S06 - BLU13] - Test database storage

[cimendes]

[S06 - BLU14] - Test database storage

Context

This is related to this issue

Detailed Description

A few students have reached the capstone without having set up the database correctly in Heroku and without noticing that no observations were being stored.

Locally, the line in this issue works but it doesn't allow Postgres to link to the app.

Possible Implementation

In BLU13, we are testing the predict/ endpoint. We can follow the same approach by asking to create a "retrieve/" endpoint that responds with the contents of the DB given an ID

[UrbanoFonseca]

Added in #43