Order of the multipolar expansion for boundary corrections.
The Poisson solvers multigrid, cg, and cg_corrected
(and fft with PoissonFFTKernel = multipole_correction)
do a multipolar expansion of the given
charge density, such that $\rho = \rho_{multip.expansion}+\Delta
\rho$. The Hartree potential due to the $\rho_{multip.expansion}$ is
calculated analytically, while the Hartree potential due to $\Delta \rho$
is calculated with either a multigrid or cg solver.
The order of the multipolar expansion is set by this variable.
Default is 4 for PoissonSolver = cg_corrected and multigrid, and 2
for fft with PoissonFFTKernel = multipole_correction.