Reintroduces the option to flush_nc_files with fms2_io

[uramirez8707]

Description
Reintroduces the option to flush_nc_files with fms2_io

This works the same way it did in 2019.01.03 but with fms2_io. If flush_nc_files is .true., it will call diag_output:diag_flush which will call fms2_io:flush_file which will call nf90_sync
Fixes #819

How Has This Been Tested?
Passes a unit test that I created.
Ran c96L33_am4p0_cmip6Diag with intel18, prod-openmp on gaea and everything still works.

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