Fixes for high-res ocean diagnostics

[alperaltuntas]

Several fixes for high-resolution ocean diagnostics:

• fixes U_eq.ncl for high-res grids.
• check if iage exists before trying to create 3d za for it.
• fix sst_eq_pac_seasonal_cycle for high-res grids.

Testing: ran the tool for a high res G case: http://webext.cgd.ucar.edu/GIAF/g.e20.G.TL319_t13.control.001/ocn/diag_work_debug.21-25/MODEL_VS_OBS/

[alperaltuntas]

Done. Thanks!