Do not extrapolate the potential beyond the right boundary.

Since the intraband kernels do not extrapolate the potential,
it makes sense to do so in the interband contribution  for
consistency
Use smaller energy grid spacing in odedirac.py
Improve kernel calculation, to avoid mishandling of oscillating
integrands
Converting rpa intraband calculation to C code
Use linear grid in intraband contributions via Kernel class
Added a switch in rpam.py to select between python and c implementations
Renamed Makefile to Makefile.linux to allow for Makefile on other OSes
Simple utilities for testing and generating kernels