[Mitgcm-support] idkey and multi-threading

[mitgcm-support at dev.mitgcm.org]

I was doing some work with Veronique and noticed the
the idkey mechanism that is used to control which
block of storage TAMC will work with in IMPLDIFF.
This mechanism will not work in a multi-threaded code.
All threads see the same idkey, even though each thread may
not be at the same instance of impldiff.

        IF (implicitViscosity.AND.momStepping) THEN
#ifdef    ALLOW_AUTODIFF_TAMC
          idkey = iikey + 3
#endif    /* ALLOW_AUTODIFF_TAMC */
          CALL IMPLDIFF(
     I         bi, bj, iMin, iMax, jMin, jMax,
     I         deltaTmom, KappaRU,recip_HFacW,
     U         gUNm1,
     I         myThid )