Add jinja2 block around memory options (#831)

* Add jinja2 block around memory options

This is to enable overriding --mem with --mem-per-gpu or other
cluster-specific options.

* Add the scoped option

See https://jinja.palletsprojects.com/en/3.0.x/templates/#block-nesting-and-scope

flow/templates/slurm.sh

@@ -4,9 +4,11 @@
 #!/bin/bash
 #SBATCH --job-name="{{ id }}"
         {% set memory_requested = operations | calc_memory(parallel)  %}
+{% block memory scoped %}
         {% if memory_requested %}
 #SBATCH --mem={{ memory_requested|format_memory }}
         {% endif %}
+{% endblock memory %}
         {% if partition %}
 #SBATCH --partition={{ partition }}
         {% endif %}