Made demand_curve method Public.

The `demand_curve` method is used to determine the peak electricity demand. In order for the result of this method to be consistent with the `loss_of_load_expectation` method, the demand curve needs to be scaled by the same demand as the one in the `loss_of_laod_expectation` and we have to be able to set the optional `demand` parameter  by a Gquery. For this to work, I made the `demand_curve` method Public.