OMM position/velocity covariance (CCSDS 502.0-B-3 table 4-3), kept exactly as read.
lower_triangle holds the 21 lower-triangle values in keyword order CX_X,
CY_X, CY_Y, CZ_X, CZ_Y, CZ_Z, CX_DOT_X ... CZ_DOT_Z_DOT: km²
for two position components, km²/s for one position and one velocity
component, km²/s² for two velocity components. cov_ref_frame is nil
when the matrix is in the OMM's REF_FRAME.