Generate trajectory even when propagating backward

As continuation of #171 (closed), fix the bug where the trajectories from backward propagations cannot be correctly queried.

Another bug, identified in #127 (closed): in the mission_design/bplane.rs test file, converting that trajectory into the Moon J2000 frame and evaluating that will lead to some states being NaN. No clue why since the ephemeris is correctly defined at those times.