Enable "CI mode" with reduced runtime for double-loop notebooks

[lbianchi-lbl]

• Currently, DL notebooks have a runtime that's too long to be run in CI
• We want to have a programmatic way to be able to set the simulation parameters programmatically so that the runtime is lower if a condition is met (e.g. myparam = 1 if running_in_ci else 10)
• This mechanism has already been implemented for the RE DL notebook, with the final setup being worked on in Update notebook checks #158
• We want to have a similar mechanism for the DL notebooks for the remaining case studies

[lbianchi-lbl]

We should revisit this when we adapt the DL-using code to the changes due to the new structure of the grid_integration package.

[ksbeattie]

This is here to remind us that we should apply this to other notebooks (when we have time)

[lbianchi-lbl]

Still no updates on this. Not sure if this'll make it to the June release, although it might be connected with #200 and possibly the intermittent failures observed in notebook CI checks on Windows in #142.