Test boundary conditions

Very rudimentary tests to verify that the boundary conditions are working as expected.

The periodic_[1,2,3]D tests test simple setups in [1,2,3] dimensions. We use a generic particle type with a constant velocity and drift it over time across periodic boundaries.

To verify the results, you can either just load the output files with paraview and manually inspect that you're not losing particles.

Alternately, a script compare_particle_positions.py is provided. It will compare the output at t=0 with the output at an end time you provide as a cmdline argument, and check whether the particles are in the positions they're supposed to be.