[R] Normalize function getting wrong index from data

[nkruskamp]

The R normalize function is not getting the correct ground Z value index when applied to the whole las data. It needs to be revised to correctly get the GROUND Z value by index.

The function subsets FULL DATA into by SUBSET ground and non-ground, and returns the index of the SUBSET of ground, not the correct index of FULL DATA where the ground point is.