Review and improve merit LV/MV/HV queries

[mabijkerk]

The queries used in the electricity network charts are often not robust, but made up out of hardcoded nodes. This makes it easy to overlook updating these queries causing them e.g., to miss part of the electricity demand.

For example, lv_households_lighting_load_curve:

    SUM_CURVES(V(
      households_lighting_efficient_fluorescent_electricity,
      households_lighting_incandescent_electricity,
      households_lighting_led_electricity,
      electricity_input_curve
    ))

This could be rewritten, to something like, as shown for buildings cooling in d7063f4:

    SUM_CURVES(
      V(
        FILTER(
          ALL(),
          "merit_order && (merit_order.group == :buildings_cooling && merit_order.level == :lv)"
        ),
        electricity_input_curve
      )
    )

Note that we might also have to change see if we can change the merit order group, or add a node group, because the lighting, appliances and cooking in households all have the same group: households_appliances.