Add POP_SUB in td_write_mxll_field

Description

Add POP_SUB before exiting subroutine when the MPI group is not root. This was failing for MPI and was not caught by the buildbots because the feature is used in a long test.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

@sohlmann @heiko.appel @martin.lueders I'd appreciate a quick review of this small MR.

requested review from @nicolastd

approved this merge request

enabled an automatic merge when the pipeline for 0bcda1fa succeeds

resolved all threads

mentioned in commit 8368731e

merged

mentioned in issue #733 (closed)

changed milestone to %14.0

added Core label

added in Changelog label