dmUpdate: move current_model_time to the field object

[rem1776]

Description moves the current_model_time variable from the main diag object to the individual field objects

I'm not too sure if the check under all fields should have the same time if writing should be there.

How Has This Been Tested? latest oneapi on amd

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] Any dependent changes have been merged and published in downstream modules
• [x] New check tests, if applicable, are included
• [x] make distcheck passes