use Matrix methods in as.matrix

[goldingn]

It may be possible to get a speed up from deterministic projections in big landscapes by using sparse matrix classes

For a 3-stage, 2000 patch dispersal model, switching from matrix to Matrix (and adding import & dealing with classes) here doesn't slow down pop:::as.matrix.dynamic() much (7.3s vs 3.9s) and results in much faster multiplications (1.2s vs 5.3s).

This only skips true 0s, so is as accurate as the dense matrix version. If the matrix construction end can be sped up, this may be worthwhile in all dispersal cases.

[goldingn]

@skiptonium definitely worth thinking about sparse matrices for dlmpr. should be easy enough to switch over to SpMat class once the whole thing is prototyped