Combine the forward and backward structures.

The packing routines could be moved to utils.hpp when they are needed elsewhere.

Member p3m_analytic_cotangent_sum (int n, double mesh_i)

Extend to higher order.

Member propagate_omega_quat_particle (Particle &p, double time_step)

implement for fixed_coord_flag

Class walberla::InterpolateAndShiftAtBoundary< FieldType, FloatType >

Currently only works for 1 MPI rank! It should work in parallel if the MPI domain decomposition for the structured block forest doesn't partition along the shear direction. For example if the shear direction goes along the z-axis, it should be possible to run on 4 MPI ranks with [2, 2, 1]. At the moment, ESPResSo requires system.cell_system.node_grid to be in decreasing order, therefore parallelization requires a shear direction along the z-axis and a MPI node_grid of [x, y, 1] with x >= y. This restriction on the ordering of the node_grid may be lifted in the distant future, when our FFT algorithm is replaced by a new one.