Added mkrpa2.py : RPA kernel

This module calculates the kernel that described RPA response
for a given grid. The function RPA_inter(r) gives the kernel
for E_F = 0, and the function RPA_intra(r, kF) calculates
the _extra_ contribution due to finite Fermi energy.
The kernels are automagically saved in data files to avoid recalculations.

The module also provides testing routines which give some insight about
the response to "topped off" Coulomb potential.
Merge branch 'closing_the_loop' of https://github.com/avshytov/Coulomb-Impurity into closing_the_loop