Disable nonrepro FMAs in convex BBL solver

Three FMAs were introduced when the BBL convex L(:) computation were
moved to a separate function.  Parentheses have been added to disable
these FMAs.

Calculation of the cell width fractions `L` was consolidated and moved
into a new function, `find_L_open_concave_analytic` (later renamed to
`find_open_L_open_concave_trigonometric`).  When compiled with Intel
and using the following flags,

    -O2 -march=core-avx2 -fp-model source -qno-opt-dynamic-align

additional FMAs are added to the function, specifically the following
expressions:

    crv_3 = (Dp + Dm - 2.0*D_vel)

    L(K) = L0*(1.0 + ax2_3apb*L0)

    L(K) = apb_4a * (1.0 - 2.0 * cos(
        C1_3*acos(a2x48_apb3*vol_below(K) - 1.0) - C2pi_3)
    )

(Third expression FMA was added inside of `acos()` by the compiler)

The crucial flag seems to be `-march=core-avx2` which controls the
introduction of FMAs and explains why the problem went undetected.

The legacy answers were restored by adding new parentheses to the above
equations.

Since this function is not generally considered reproducible and is
provided for legacy support, the loss of performance is not considered
to be an issue.

src/parameterizations/vertical/MOM_set_viscosity.F90

@@ -1178,7 +1178,8 @@ subroutine find_L_open_concave_trigonometric(vol_below, D_vel, Dp, Dm, L, GV)
 
   ! Each cell extends from x=-1/2 to 1/2, and has a topography
   ! given by D(x) = crv*x^2 + slope*x + D_vel - crv/12.
-  crv_3 = (Dp + Dm - 2.0*D_vel) ; crv = 3.0*crv_3
+  !crv_3 = (Dp + Dm - 2.0*D_vel) ; crv = 3.0*crv_3
+  crv_3 = (Dp + Dm - (2.0*D_vel)) ; crv = 3.0*crv_3
   slope = Dp - Dm
 
   ! Calculate the volume above which the entire cell is open and the volume at which the
@@ -1205,10 +1206,13 @@ subroutine find_L_open_concave_trigonometric(vol_below, D_vel, Dp, Dm, L, GV)
       !   vol_below(K) = 0.5*L^2*(slope + crv/3*(3-4L)).
       if (a2x48_apb3*vol_below(K) < 1e-8) then ! Could be 1e-7?
         ! There is a very good approximation here for massless layers.
-        L0 = sqrt(2.0*vol_below(K)*Iapb) ; L(K) = L0*(1.0 + ax2_3apb*L0)
+        !L0 = sqrt(2.0*vol_below(K)*Iapb) ; L(K) = L0*(1.0 + ax2_3apb*L0)
+        L0 = sqrt(2.0*vol_below(K)*Iapb) ; L(K) = L0*(1.0 + (ax2_3apb*L0))
       else
+        !L(K) = apb_4a * (1.0 - &
+        !         2.0 * cos(C1_3*acos(a2x48_apb3*vol_below(K) - 1.0) - C2pi_3))
         L(K) = apb_4a * (1.0 - &
-                 2.0 * cos(C1_3*acos(a2x48_apb3*vol_below(K) - 1.0) - C2pi_3))
+                 2.0 * cos(C1_3*acos((a2x48_apb3*vol_below(K)) - 1.0) - C2pi_3))
       endif
       ! To check the answers.
       ! Vol_err = 0.5*(L(K)*L(K))*(slope + crv_3*(3.0-4.0*L(K))) - vol_below(K)