Added equinoctial elements. Also a little cleanup

readme.md

@@ -10,9 +10,9 @@ also differentiate.
 
 The crate is still being built to a milestone plan:
 
-- **M1** — the study-critical subset: two-body and spherical-harmonic gravity,
-  drag, third-body, cannonball SRP, GCRF/ITRF frames, ground stations and the
-  four measurement types, with a validation suite against Orekit reference
+- **M1** — the core subset: two-body and spherical-harmonic gravity, drag,
+  third-body, cannonball SRP, GCRF/ITRF frames, ground stations and the four
+  measurement types, with a validation suite against Orekit reference
   trajectories.
 - **M2** — general-purpose completion: higher-degree gravity fields, more
   density models, box-wing SRP and eclipses, maneuvers, covariance propagation,