Optional OPM 6x6 covariance, held 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. No
symmetry or definiteness check is applied, so a matrix that falls short of
positive semidefinite only through the digits it is printed to is read.
to_matrix/1 expands it to six symmetric rows.
Summary
Functions
The symmetric 6x6 matrix, as six six-element rows, that the 21 lower-triangle values state. The values are placed as read; nothing is validated.