This commit wraps up the implementation of sugar loading on NUE. It contains several intrusive changes: (1) several bug fixes related to leaf_age and leaf_frac in stomate_turnover, (2) commented the water stress (temp_w) on leaf turnover (in stomate_turnover) as it looks like the current approach decreases the turnover when there is enough water (=unexpected) and increases the turnover when there is too little water (expected), (3) commented the calculation of fruit turnover at the end of stomate_turnover because it is already calculated when dealing with leaf and root turnover earlier in the same subroutine, (4) replaced the element flow between the labile and reserve pool but a much simpler approach. This has a substantial impact on the simulations, and (5) externalized root_reserve which helped to remove some minor inconsistencies in its calculation.
