sdc_raster for large rasters is slow

[edwindj]

sdc_rasters rasters with more than 1000x1000 cells take minutes to create.

Typically city size rasters are fast (<second), country sized raster slow (minutes).

This is due to:

• overly generic use of geographical objects: points x,y are to be rastered on a square grid, but are first translated into WKT (by raster) and then rasterized.
• the calculation for the internal attacker value makes it slow, for each cell a R function is executed. This can be improved

The operation can be made a lot faster: seconds for a country sized raster.