HollowCone fix

[jngrad]

Fixes espressomd/espresso/#2129

Applied a fix by @RudolfWeeber and corrected the cone orientation in the tutorial.

Note: during the tutorial simulation exercise, output files *.dat and *.vtk store the negative of the particle velocities, such that the curves of Figure 6 are flipped on the Y-axis when plotted in a spreadsheet software and the velocity arrows point in the opposite direction in ParaView.

[RudolfWeeber]

@mkuron, could you please try the rectification simulation with those fixes? THx

[mkuron]

@jngrad needs to merge jngrad#1, then @hmenke needs to merge the present pull request (#4). @hmenke then needs to resolve a bunch of merge conflicts and apply the style changes suggested by clang-format. Then espressomd#2129 can be merged.