mHM routine: Vmin <= Vn

[casadoj]

The calibration of the mHM routine produces in some instances reservoirs whose minimum storage ($V_{min}$) is larger than the normal storage ($V_n$), which is nonesense.

This error is produced by the definition of $Vn$ as a factor of $V{tot}$, where $\gamma$ is a calibration parameter with a search range from 0 to 1:

$$Vn = \gamma \cdot V{tot}$$

In the current implementation, I fix $V_{min}$ to the minimum storage in the calibration period. That means that it is never checked that $Vn$ must be equal or larger than $V{min}$.

[casadoj]

Close #8

I have modified the __init__ method in the class mHM to update $V_{min}$ depending on $\gamma$.

Vmin = min(Vmin, gamma * Vtot)