Added equinoctial elements. Also a little cleanup

src/frames.rs

@@ -38,7 +38,7 @@ pub struct Frame {
 }
 
 impl Frame {
-    /// Earth-centered, J2000-oriented. The frame the study's scenarios work in.
+    /// Earth-centered, J2000-oriented.
     pub const EARTH_J2000: Self = Self {
         center: Body::EARTH,
         orientation: Orientation::J2000,