Incidental failure of the tests of the tangent-linear and adjoint versions of the vertical-tracer-mixing subroutines

[sam]

The results of the tests of the tangent-linear and adjoint versions of subroutines 'tra_zdf_exp' and 'tra_zdf_imp' have been found to switch between success and failure when a seemingly-unrelated model option is toggled (compiling with and without 'key_mpp_mpi').

Examination of the test subroutines 'tra_zdf_exp_adj_tst' and 'tra_zdf_imp_adj_tst' revealed that the array 'r2dtra' remains undefined prior to being used in the tangent-linear and adjoint versions of the subroutines that compute the vertical tracer mixing. The test subroutines are independently implemented for the optional time stepping schemes (split-explicit or implicit), and the tangent-linear and adjoint versions of the specific implementations are called directly by the tests. In contrast, as part of the regular time stepping of the tangent-linear or adjoint model, the selected implementation is called via subroutine 'tra_zdf_tan' or 'tra_zdf_adj', which both define the array 'r2dtra', and so this bug does not occur during the regular tangent-linear or adjoint time step.