coarse_graining function

[bniebuhr]

I suggest we implement a function in ConScapeR equivalent to the coarse_graining function in ConScape.jl. Ideally this should be implemented in Julia and without the need to use the grid object (directly from the (target) habitat quality map).

For the time being, one possible solution was implemented here in 68233a6. It is probably not a good one, but it works while another solution is not put in place.