ENH: new Flight.get_solution_at_time() method

[Gui-FernandesBR]

Pull request type

• Code changes (bugfix, features)

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

If by any chance you want to know the state vector that corresponds to a specific time of the flight, there's no easy way to do so.

New behavior

There's this new method that returns the numpy array of the solution state at a specific time. If the user choose a time that do not exists in the timesteps, a warning is raised and the code automatically changes to the closest time step.

Breaking change

• No

Additional information

• This feature is particularly important for us to work on multi-stage and deployable flights.
• I was not sure if it was better to return the np.array object or a list. I chose array because I thought it could be more useful and faster, but honestly this was arbitrary and I don't think there is a big difference between the two options.
• as I am creating this PR on windows, I wonder if the integrators of the Flight class will give the exact same behavior on linux when I upload this to github actions. Let's see!