Add a "demand" parameter to loss_of_load_expectation

Since one graph cannot read values from another (i.e. future graph
reading a property of the present graph), a "demand" parameter has been
added to loss_of_load_expectation so that the demand can be calculated
elsewhere, and then passed into the calculation.

For example:

  # LOLE Demand Query

  V(GRAPH(), final_demand_for_electricity) +
    V(GRAPH(), energy_sector_final_demand_for_electricity) +
    QUERY_PRESENT(-> { V(energy_power_hv_network_loss, demand) })

  # LOLE Calculation

  V(GRAPH(), loss_of_load_expectation(
    Q(lole_demand), Q(reliable_electricity_production_capacity)))