Fill NodeData.water_flux when using Bucket scheme

Currently, commit 37109aa1 implements a NodeData.water_flux variable to document the water fluxes across the nodes. This variable is only filled when using Richards Equation, while it is kept at 0. when using the Bucket scheme. It must be pretty easy to also store the water fluxes across nodes (i.e., between elements) when using the Bucket scheme (in WaterTransport.cc). This could be necessary or useful when doing runoff modeling in Alpine3D. Runoff variables in Alpine3D should be correct now when at least soil is solved with Richards Equation.