Ensure catfiles (compiler output, etc) is added to final output when using slurm prediff.
Merge pull request #357 from thomasvandoren/update-slurm-prediffer

Ensure catfiles (compiler output, etc) is added to final output when using slurm prediff.

This fixes an error where the output file generated by the compiler was not included in the final output,
and resulted in mysterious failures. I think the bigger issue is that catfiles are evaluated before the
system prediff. I am not yet sure if there is a reason for that, but this change is useful regardless.