Separate subroutines for land cover change and wood use. Done by S. Luyssaert, see ticket #99
No change in results if VEGET_UPDATE is kept constant all the time from the first start without restart file. These changes correct the behaviour when first using VEGET_UPDATE=1Y followed by VEGET_UPDATE=0Y. In the new version, the wood in product pool will continue to decompose even when VEGET_UPDATE=0Y.