Changeset 2772 for branches

Bug corrections :

• The depth of the interface between the vertical level did not take into account the special cases of the first and last levels which exists in the hydrology.
• The precision of the time calculation was improved in forcing_tools to avoid some spurious behaviour in the interpolation between forcing time steps. This could not be applied to solarang as it produces too large steps in solar radiation when transitioning between one forcing time steps.