Brouwer-Lyddane Critical Inclination Fix
I saw that the Brouwer-Lyddane model was added in #653 (closed) and it is not applicable for critical inclinations. Warren Phipps' 1992 thesis at [1] documents a fix for the critical inclination problem. It does change the behavior of the propagator near the critical inclination so perhaps it is worthwhile to keep the original Brouwer-Lyddane model as well as one closer to the PPT2 model which is applicable for all near Earth orbits.