For finite systems, some Poisson solvers (multigrid,
cg_corrected, and fft with PoissonFFTKernel = multipole_correction)
require the calculation of the
boundary conditions with an auxiliary method. This variable selects that method.
A multipole expansion of the density is used to approximate the potential on the boundaries.
An exact integration of the Poisson equation is done over the boundaries. This option is
experimental, and not implemented for domain parallelization.