Export trajectory addition and frame conversion to Python

[ChristopherRabotin]

Effects

Allows for this from Python:

    traj1_moon = traj1.to_frame("Moon J2000")
    traj2_moon = traj2.to_frame("Moon J2000")

    traj_moon = traj1_moon + traj2_moon

This feature was already available in Rust.

Breaking changes

1. The addition operator in Rust now returns a Result type after checking for the frame. Prior to this PR, it was possible to add two trajectories in different frames.
2. The order of addition matters: with self+rhs, only the states whose epoch is greater than self.last().epoch are added to the trajectory, so if there is an overlap in epochs, the order of the addition matters.

If this is a new feature or a bug fix ...

• Yes, the branch I'm proposing to merge is called issue-xyz where xyz is the number of the issue.

If this change adds or modifies a validation case

• No.

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