Started correct perp. RKKY coupling

The perpendicular coupling only along the z-axis is not correct.
One has to take all lattice vectors into account which are inside
a sphere of radius Rmax. This includes vectors with non-vanishing
perpendicular AND parallel components.

Here, I started to implement the infrastructure for this. In
particular the perpendicular Green's function is stored in
apcu[n][m][i][j], with

n: 0,...,Npp-1

	where Npp=N or Npp=2*N+1, depending on mirror symmetry)

m: 0,...,Npp_size[n]-1

	where Npp_size[n]=Npp_end[n]-Npp_start[n]+1
	and     Npp_start=max(n-2*Rmax,0)
	and       Npp_end=min(n+2*Rmax,Npp-1)

i: 0,...,N[n][m]-1

	where N[n][m]=rpnn(abs(n-m-Npp_start[n])).size()

j: 0,...,agrid.M

rpnn[k] holds the distances of the
nearest neighbors inside a monolayer which is k=n-m layers away
from the origin (see neighbors.py from euoscripts).