Added the MEKE_POSITIVE parameter to enforce MEKE to be positive

[Wendazhang33]

This commit added a new parameter MEKE_POSITIVE to deal with the issue that MEKE becomes negative. If MEKE_POSITIVE is True (default is False), all negative values of MEKE will be set to zero. This operation will break the conservation of MEKE so should be used carefully. The motivation to add this parameter is that we found the MEKE simulation became unstable in the global coupled experiment of ESM4.5. The instability was caused by the negative values at a few shallow regions (depth smaller than 10 m). These negative values quickly spread into the open ocean and made MEKE negative over the whole ocean. By using MEKE_POSITIVE to clip out negative values, the source of instability was removed and the simulation seemed to be sensible. Although this method violates the MEKE conservation, negative values seemed to mainly happen at a few shallow regions, which may not cause a big error to the global MEKE conservation.

The best solution for this issue would be to fix the bug that causes negative values in the MEKE module. There are a few terms, such as MEKE%mom_src and MEKE%GM_src, that are not positive definite. Fixing these may resolve the negative MEKE issue, but I have not had a success yet.

[Hallberg-NOAA]

This capability (with an identical name) was already added to MOM6 via NCAR#279, which was merged onto the main branch of MOM6 as a part of mom-ocean#1653. As such, this useful contribution to MOM6 is no longer needed.