Provide ground heat flux to potential_ET, if its part of the data.frame

Currently, potential_ET ignores the ground heat flux G (and S) when called with data.frame where this column is present. (See e.g. `potential.ET(tha)` from the vignette. Although this is consistent with the section in the vignette, this might be unexpected to the user.

I suggest to change the default behavior to check if this column (G or S) exist and if so, use this column in the computation.