sparse->dense coercion

[doliv071]

I've been using cellXY and it is quite nice and works painlessly. I would like to make a suggestion, to remove all as.matrix operations if possible. It browsing the code it doesn't seem like there is any requirement that the matrix be dense as all lower level functions seem to handle sparse matrices just fine.

For a decent sized experiment this operation might be enough to crash smaller machines.

In asMethod(object) :
  sparse->dense coercion: allocating vector of size 16.7 GiB

Best,
Dave