o change calculation of Visbeck K:   - no longer depend on tapering scheme; instead, compute slope (limited      by GM_maxSlope). this is equivalent to what was comuted before with     clipping or linear tappering.   - fix vertical averaging (wrong by 1 level)   - leave the old version (but will not be maintained) within     #ifdef OLD_VISBECK_CALC / #endif
o change calculation of Visbeck K:
  - no longer depend on tapering scheme; instead, compute slope (limited
     by GM_maxSlope). this is equivalent to what was comuted before with
    clipping or linear tappering.
  - fix vertical averaging (wrong by 1 level)
  - leave the old version (but will not be maintained) within
    #ifdef OLD_VISBECK_CALC / #endif

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