The PlantsModel is pretending to calculate evapotranspiration

[davidorme]

At the moment in #659 (PR pending), the PlantsModel is calculating actual transpiration per grid cell in mm m-2 but is writing that to the data["evapotranspiration"]. As discussed with @vgro, we think the best thing to do is make the data needed to calculate canopy evaporation available to the Abiotic model. For the moment though, while PlantsModel is being reworked, we'll just accept the error.