Optional OPM Keplerian elements.
anomaly is a tagged tuple, either {:true_anomaly, deg} or {:mean_anomaly, deg}, preserving which anomaly keyword the message carried.
anomaly
{:true_anomaly, deg}
{:mean_anomaly, deg}
@type anomaly() :: {:true_anomaly, float()} | {:mean_anomaly, float()}
@type t() :: %Sidereon.CCSDS.OPM.Keplerian{ anomaly: anomaly(), arg_of_pericenter_deg: float(), eccentricity: float(), gm_km3_s2: float(), inclination_deg: float(), ra_of_asc_node_deg: float(), semi_major_axis_km: float() }