Cartesian state's add_unchecked does not set the frame correctly

[ChristopherRabotin]

Bug report

Describe the bug

As part of nyx-space/nyx#86, I'm testing the spacecraft trajectory conversions. In the traj_spacecraft test, I convert a spacecraft propagated in the EME2000 frame to the Moon frame and back again into the EME2000 frame. The returned value is obviously wrong indicating that the round-trip frame translation in Earth/Moon J2000 adds E-M vector.

left: Spacecraft { orbit: CartesianState { radius_km: [[-7273.342668443073, 253.99072179207016, 0.022164871880207732]], velocity_km_s: [[-0.2582852231926599, -7.396321042416258, -0.0006454507752619773]], epoch: 2021-01-01T12:00:00 UTC, frame: Frame { ephemeris_id: 399, orientation_id: 1, mu_km3_s2: Some(398600.435436096), shape: Some(Ellipsoid { semi_major_equatorial_radius_km: 6378.14, semi_minor_equatorial_radius_km: 6378.14, polar_radius_km: 6356.75 }) } }, dry_mass_kg: 300.0, fuel_mass_kg: 67.0, srp: SrpConfig { area_m2: 0.0, cr: 1.8 }, drag: DragConfig { area_m2: 0.0, cd: 2.2 }, thruster: Some(Thruster { thrust_N: 0.089, isp_s: 1650.0 }), mode: Thrust, stm: None }

right: Spacecraft { orbit: CartesianState { radius_km: [[234377.09135147667, -262800.33219000406, -143205.15698775538]], velocity_km_s: [[0.5099871196829026, -6.7531442107201975, 0.21484133260643304]], epoch: 2021-01-01T12:00:00 UTC, frame: Frame { ephemeris_id: 399, orientation_id: 1, mu_km3_s2: Some(398600.435436096), shape: Some(Ellipsoid { semi_major_equatorial_radius_km: 6378.14, semi_minor_equatorial_radius_km: 6378.14, polar_radius_km: 6356.75 }) } }, dry_mass_kg: 300.0, fuel_mass_kg: 67.0, srp: SrpConfig { area_m2: 0.0, cr: 1.8 }, drag: DragConfig { area_m2: 0.0, cd: 2.2 }, thruster: Some(Thruster { thrust_N: 0.089, isp_s: 1650.0 }), mode: Thrust, stm: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After some digging, it was quickly obvious that the add_unchecked function does not set the frame. Therefore, a round trip using translate_to or transform_to will be incorrect!

To Reproduce

1. Checkout the Nyx repo on the correct branch
2. Run cargo test --release traj_spacecraft
3. Test fails

Expected behavior

Round trip conversions should always work all the time.

Platform

Linux, Rust