fix extra headers formatting in slurm script

[d3sm0]

It seems that in some cases the extra_slurm_header is not well formatted as a result sometimes we get:

#!/bin/bash

#SBATCH --mem=24GB
                 #SBATCH --cpus-per-task=4

Instead we can force a list of string each of them containing an sbatch argument, strip each item and write it in the appropriate row.

#!/bin/bash

#SBATCH --mem=
#SBATCH --cpus-per-task=4