BUG: Maintaing Extrapolation when Adding Discrete Functions with Constants

[MateusStano]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)
• ReadMe, Docs and GitHub updates
• Other (please describe):

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest --runslow) have passed locally

Current behavior & New behavior

When adding a constant value to a discrete Function, the extrapolation method is not maintained. This PR fixes this issue

[Gui-FernandesBR]

Well done, @MateusStano !

Did you also conder other occurrencies of the same behavior in the same class?
Please try to taka a look at the following code lines too: 1720* ,1722, 1743, 1746, 1789, 1851, 1869, 2039*, 2054, 2329.

• Specially the 1720 and 2039, I think they are the most important ones.