These routines are wrapper around the Poisson solver of BigDFT. 
It is used to compute the convolution with 1/r and to compute 
the exchange and correlation related stuffs (potential, energies...).

To do it, one must use a kernel. 
Create it with psolver_kernel() (iaction == 2 to get it and build it if not already created). 
Then it can be used in psolver_rhohxc()) that is the equivalent with free boundary conditions to rhohxc(). 
It can also be used in psolver_hartree() which only does the convolution and does not compute XC.
