Check dependence on sp package

[LHMarshall]

sp and various other spatial packages are being phased out.

Check and remove dependence on these packages.

[LHMarshall]

Dependence on sp summary:

importFrom(sp,Polygon)
importFrom(sp,Polygons)
importFrom(sp,SpatialPolygons)

calc.perp.dists - done
calc.rad.dists- done
generate.pop.D done
generate.pop.N- done
get.density.surface- done
Population- done
Survey- done