Add an option to apply user-supplied weights

Related to inv_utils.f90:

     42     WEIGHTS(:)=NOISE
     43     !-----------------------------------------------------------
     44     WEIGHTS(1)=(2.0*(1.0+4.0*RATUMQS-ICVAL/ICMEAN)/(1.0+RATUMQS*ICVAL/ICMEAN))
     45     IF (WEIGHTS(1) > 2.5) WEIGHTS(1)=2.5
     46     IF (WEIGHTS(1) < 0.5) WEIGHTS(1)=0.5
     47     WEIGHTS(1)=WEIGHTS(1)*NOISE

Try to get this out of Fortran into Python