Add Payu init script for `diag_table` generation

[minghangli-uni]

When using access-experiment-generator, updating diag_table after creating separate branches with access-experiment-runner is a bit cumbersome.

Currently, users need to manually run:

python3 make_diag_table.py

each time new branches are created.

Add a Payu init script that automatically runs make_diag_table.py after branches are created. This will streamline the workflow and remove the need for manual execution.

This feature is not intended for production use but is a convenient practice for testing and development.