Include DR in calculation of reserve requirements

[PascalLesage]

Reserve requirements can be calculated as a function of load (e.g. 3% of load at given timepoint for spinning reserves).
The load is taken from system_load, and does not account for DR that actually decreases the load.
It would be elegant to consider only net load, after DR. OF course, we would need to know what projects are DR in order to include their output in the calculation of reserve requirements.
Perhaps another solution would be to make these projects provide reserves (e.g. give them a non-nul spinning_reserves_ramp_rate=1) but this seems less elegant.