Add option to the Grass resource to be grouped

In TC 2.5.0 and before the Grass resouce has the following syntax:

Grass(Block,Data,Frequency,Rarity,BlockSource[,BlockSource2,BlockSource3.....])

In TC 2.5.1, this became:

Grass(PlantType,(unused
argument),Frequency,Rarity,BlockSource[,BlockSource2,BlockSource3.....])

(PlantType is one of the predefined plant types, or simply Block:Data)

Now the syntax is:

Grass(PlantType,Grouped/NotGrouped,Frequency,Rarity,BlockSource[,BlockSource2,BlockSource3.....])

TC 2.5.2 can still read all syntaxes, but it can only write the newest.