Fix mingen issues with several modifications

[flomnes]

1. [X] Remove truncation from weekly turbine credits (was done previously)
2. [X] Always enable monitoring of stock levels (including overflow)
3. [X] Take overflow into account in the "HydroPower" constraint (was done in 9.1.0)
4. [X] Remove recomputation of reservoir levels & overflow
5. [X] Correct infeasibility in hydro heuristic problem, related to mingen > 0