* 2.6:
  changed version number to 2.6.1
  corrected default value of selection_magnification_bias=0
  corrected a factor in the definition of C_l^shearshear
  removed a memory leak in functions raw_cl ans lensed_cl
  removed a memory leak in functions pk and pk_lin of the wrapper
  added to the wrapper new functions f() and sigma()
  corrected bugs affecting only P_NL(k) in presence of isocurvature modes