Intelligent, sometimes with a stern aura, yet attractive, warm and inviting, Michael Learned was a stage actress virtually unknown to TV viewers when she was cast as Olivia Walton, mother of John-Boy and the others, in the long-running CBS series "The Waltons" (1972-81). Assuming the role that Patricia Neal had originated in a TV-movie special, Learned was so unknown that she was originally billed as "Miss Michael Learned" lest the audience be confused as to her gender and about which part she was playing. A mother of three by the time she was 24, Learned had honed her craft primarily at the