MODERN_DIAG_MANAGER: Fix cell measures attribute

[uramirez8707]

Description
This PR fixes an issue when adding the cell_measures attribute. The code was not getting the output name of the area variable correctly.

FMS/diag_manager/fms_diag_field_object.F90

Lines 816 to 822 in 59763e9

	!< If writing the varname can be the outname which is defined in the yaml
	if (present(to_write)) then
	if (to_write) then
	!TODO this is wrong
	rslt = this%diag_field(1)%get_var_outname()
	endif
	endif

Fixes #1814

How Has This Been Tested?
CI, including new test (which fails without this update)

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

[rem1776]

lol