Full implementation of vegetation-climate relationships

[dschlaep]

• vegetation inputs by plant function consist of: relative fraction, height, mean monthly biomass components, rooting distribution
• currently, fraction (composition)-climate relationships are available in SOILWAT2
• biomass-climate relationships are only available in rSOILWAT2 (see rSOILWAT2::estimate_PotNatVeg_biomass())
• root distribution relationships are only available in rSOILWAT2 (see rSOILWAT2::estimate_PotNatVeg_roots())

--> implement everything in SOILWAT2 (and backport access in rSOILWAT2, see rSOILWAT2::estimate_PotNatVeg_composition() and rSOILWAT2:::estimate_PotNatVeg_composition_old())