Bug in Dt estimation

[szaghi]

The algorithm for estimating the time step Dt is bugged, see https://github.com/szaghi/OFF/blob/testing/src/Data_Type_SBlock.f90#L1300

The speed of sound is not multiplied for the interface area thus the vmax is underestimated. Corrected computation should be something like

        ! computing the local speed of sound
        ss = a(p=block%C(i,j,k)%P%p,r=block%C(i,j,k)%P%d,g=block%C(i,j,k)%P%g)
        ! evaluating the maximum propagation speed of acoustic segnals multiplied for face area
        ! left i
        vm   = 0.5_R_P*(block%C(i-1,j,k)%P%v+block%C(i,j,k)%P%v)
        vmiL = (abs(vm.dot.block%Fi(i-1,j,k)%N) + ss ) * block%Fi(i-1,j,k)%S