Fertilization for seed drills

This specialization adds under-root fertilisation to sowing machines. 

Features V4 SoilMod-Edition (requires SoilMod v1.2.x !!): 

• separate fertilizer tank
• enable/disable fertilization
• change fertilizer type (A/B/C)
• halfside shutoff for realistic creation of driving lines
• separate use of ridge markers
• Instructions included