El kinetic kd test case

Starting point for a benchmark for comparing different path generation methods.

• The executable eiron-path-distribution-test traces paths and writes their location counts (divided by the number of trajectories at fixed intervals dt to a grid.
• The script run-path-comparison.sh repeatedly calls this executable for differing number of particles with for two different seeds. It then calls eiron-compare-output to compare the results. We expect the MSE to decrease inverse linearly with the number of samples.

Issues before merging:

• eiron-compare-output crashes, comparison needs to be done another way. @lapposka , you showed me how to do this last week, but I lost the commands. Can you document these here?
• I'm not good enough with CMake to get the script to copy to the equivalent folder in the build directory automatically. Again, @lapposka I could use some help here.

Next steps:

• Do we have a good value now for delta_t? This should be tested after everything works.
• Using this benchmark to compare KDMC and kinetic simulation.