Limiting of MYNN2.5 for QKE heating.

[AMLattanzi]

This PR tries to address the singularity with MYNN2.5 PBL model. We can either add a QKE_e variable and compute the equilibrium value when making the QKE sources, or we can compute it on the fly. The approach taken here follows the latter.

1. Remove the eddyDiffs setVal(0.0) each time step so we have the old K_turb available in the PBL module. We now only do a setVal(0.0) when creating the eddyDiffs and overwrite the data each step.
2. Compute QKE_e from a balance of shear production, buoyancy production, and dissipation. Do appropriate limiting since gradients may be near machine precision and solution for QKE_e^2 becomes negative.
3. Use QKE_e to compute 1-\alpha from HL 88 paper