Slab neighborhood search is broken

[jngrad]

The system.analysis.nbhood(pos=None, r_catch=None, plane='3d') function has two search mode: spherical region (default) and slab region. The slab search is incorrect, because the bitmask is inverted. Instead of searching for particles within a distance r_catch of a plane, it searches for particles in a cylinder of radius r_catch perpendicular to the plane. There is no python test for the slab search mode. The slab search mode was already incorrect in the Tcl version of ESPResSo, and it doesn't take Lees-Edwards boundary conditions into account.

MWE:

import espressomd
import numpy as np
system = espressomd.System(box_l=3 * [10])
origin = np.array([10., 10., 0.])
offset = 4.
search_dist = 1.
system.part.add(pos=origin + [0., 0., 0.]) # inside sphere, distance to center is 0
system.part.add(pos=origin + [0., 0., offset]) # inside cylinder, distance to z-axis is 0
system.part.add(pos=origin + [offset, 0., 0.]) # inside slab, distance to xy-plane is 0
print(system.analysis.nbhood(origin, search_dist, plane='3d'))
print(system.analysis.nbhood(origin, search_dist, plane='xy'))

Output:

[0]
[0, 1]

Expected output:

[0]
[0, 2]

Visual inspection (only particles in the neighborhood appear in the visualizer):

import espressomd.visualization
system = espressomd.System(box_l=[10] * 3, time_step=0.1)
system.cell_system.skin = 0.
for i in range(10):
    for j in range(10):
        for k in range(10):
            system.part.add(pos=[i,j,k])

ids = system.analysis.nbhood([5., 5., 5.], 4.5, plane='xy')
coords = system.part.by_ids(ids).pos
system.part.clear()
for pos in coords:
    system.part.add(pos=pos)

espressomd.visualization.openGLLive(system).run(0)