[gaia][feature] Closes #869: vegetation included in sediment transport
- work realized during Jiaze Li's PhD (under the supervision of Pablo Tassi) at EDF R&D
- new keywords: VEGETATION, EFFECTIVE PLANTS HEIGHT, PLANTS STEM DIAMETER, NUMBER OF STEMS PER UNIT AREA, AREAL DENSITY OF PLANTS
- update of user documentation
- new test case: vegetation-t2d