OPAL_1997_S3396100: fix
Replace x_p (= p_baryon/p_beam) by x_E (= E_baryon/p_beam). Correct the definition of Log(1/x_p) to match the definition in the experimental paper.
Replace x_p (= p_baryon/p_beam) by x_E (= E_baryon/p_beam). Correct the definition of Log(1/x_p) to match the definition in the experimental paper.