Cell measures final fix

[uramirez8707]

Description
This is completely fixes the cell measures issues from previous versions of FMS

The problem was that the var_output_name variable was not being defined whenever this if statement is true.

FMS/diag_manager/fms_diag_field_object.F90

Line 2023 in df64d67

if (index(att, field_name) .ne. 0) return

This will happen if multiple variables have the same area. The first time var_output_name will be defined correctly, but the second time that if statement will be true, leading to undefined behavior.

Fixes #1842

How Has This Been Tested?
CI, including new test (the test fails without this update as expected)
ESM4.5 has been running with the 6f9ad52 commit

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]

lgtm