add example to costrasterGen that uses a SpatialPointsDataFrame as input. Fix non-working code below
#'#using a SpatialPointsDataFrame
#'xymat<-matrix(3,3,nrow=1,ncol=2)
#'xymat<-data.frame(xymat)
#'names(xymat)<-c("x","y")
#'xyspdf<-SpatialPointsDataFrame(xymat,data=xymat)
#'costras<-costrasterGen(xyspdf,pols,extent="points",projstr=NULL)

in errorGen:
trim mvp by complete.cases

add warning to pathdistGen about "unable to allocate memory" error
solution is to change the chunksize

use ".Machine" to specify the largest value that path distances can take
