o vectorize salt_plume package:   - add extra code to salt_plume_frac, so that i-loop vectorizes   - add code to avoid divisions by zero   - efficiency of salt_plume_frac is still sub-optimal, but acceptable
o vectorize salt_plume package:
  - add extra code to salt_plume_frac, so that i-loop vectorizes
  - add code to avoid divisions by zero
  - efficiency of salt_plume_frac is still sub-optimal, but acceptable

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h