Check implicit bottom friction

[jasontempestholt]

Currently Implicit bottom friction is only activated in dynspg_ts with wetting drying because of bit comparison issue. May help bottom friciton stability issues

[jasontempestholt]

In current version of dynspg_ts.F90

Line 648
! Add bottom stresses:
!jth do implicitly instead
IF ( .NOT. ll_wd ) THEN ! Revert to explicit for bit comparison tests in non wad runs
DO_2D( 0, 0, 0, 0 )
zu_trd(ji,jj) = zu_trd(ji,jj) + zCdU_u(ji,jj) * un_e(ji,jj) * hur_e(ji,jj)
zv_trd(ji,jj) = zv_trd(ji,jj) + zCdU_v(ji,jj) * vn_e(ji,jj) * hvr_e(ji,jj)
END_2D

Line 711
!jth implicit bottom friction:
IF ( ll_wd ) THEN ! revert to explicit for bit comparison tests in non wad runs
DO_2D( 0, 0, 0, 0 )
ua_e(ji,jj) = ua_e(ji,jj) / ( 1._wp - rDt_e * zCdU_u(ji,jj) * hur_e(ji,jj) )
va_e(ji,jj) = va_e(ji,jj) / ( 1._wp - rDt_e * zCdU_v(ji,jj) * hvr_e(ji,jj) )
END_2D
ENDIF

[jasontempestholt]

Stability criteria is:

U<H/(DT.Cd)

So only an issue in shallow water, but

10/(900x0.01)=1
So care may be needed