Fixing calories tracker bugs, removing debugging prints, applying PR 24 and PR 56

[ahmedibrhm]

• [x] fixing calories tracker bug. It was divided by zero if there is no chosen diet.
• [x] inquiring by time difference for getting the user history instead of the number of times. A user doesn't have to get a meal every day.
• [x] removed print statements that were used for debugging
• [x] added PR 24
• [x] added PR 56

Sorry for the many edits but it's the easiest way to keep the two branches parallel. I check the tests before each new one. It's just I have this local branch connected directly to the deployment branch and the PR takes time to be reviewed.

[Shadimrad]

That's great! tests are passing.