Let dam_lreservoir consider precipitation and evaporation when trying to keep the allowed water level drop? #204

While working on #203, I realised that dam_llake and dam_lreservoir calculate the allowed discharge (which prevents the water level from dropping too fast) differently (see #203 (comment)). dam_llake includes precipitation and evaporation (Calc_AllowedDischarge_V1 and Update_AllowedDischarge_V1) while dam_lreservoir does not (Calc_AllowedDischarge_V4 and Update_AllowedDischarge_V2). I see no reason why they should not be included.

I don't think the difference is likely to significantly affect simulation results under realistic settings. And, of course, the control of real dams isn't precise enough to accurately account for evaporation losses. But it is still a small logical flaw and inconsistency. Hence, I tend to suggest adjusting dam_lreservoir and including this adjustment in HydPy 7.0. (For HydPy 6.5, I would prefer to avoid even the slightest change in simulation results unless it fixes a real bug.)