add root pe number to diag_field_log filename

[rem1776]

Description
adds the root pe number to the filename of the the diag_field_log.out. When running SPEAR it seems like mom was writing from a different root pe and overwriting the file, so this avoids that by writing out distinct filenames.
Fixes #1496

How Has This Been Tested?
tested with the spear regressions, intel23-classic

Checklist:

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

[bensonr]

There are still opportunities for race conditions of two MPI-ranks writing to this file at the same time. I think we'd be better off using the same approach as logfile.out and appending the root_pe number to the file to make them unique.