Basic KDMC functionality

Overview of changes:

• Added a KDSimulator struct to simulator.hpp and simulator.cpp.
• Added a file KD_simulation.cpp that contains the routines simulate_particle_from_source and follow_particle, corresponding to the names already used in the code for kinetic simulations.
• app/eiron-path-distribution-test/main.cpp now has a new command-line argument selecting which time stepping producedure to use. Depending on this argument, either a classic kinetic simulation is started or a KD simulation is started. The resulting quantity being tallied on the grid bi this executable is a time step delta_t multiplied by particle counts in the grid cells, evaluated at intervals delta_t.
• Added a script that calls the eiron-path-distribution executable for both schemes with varying values of delta_t. For each delta_t, the two results are compared using the grid_diff tool.
• Modified CMake configuration to add the above changes to the build.
• Added a placeholder test-case that always fails for now.

Still to be done before merge:

• Remove commented out CMake lines used to speed up build during debugging
• Move KD tallying out of timestepping as the current code runs out of memory for delta_t < 0.01 on my machine
• Add some basic tests and document whether the simulation results make sense.
• Merge in from master

Future work:

• Refactoring and cleanup of the code to match remaining codebase
• More extensive testing
• Dealing with ionization during diffusion properly
• Extend KDMC to deal with other simulation types
• Diffusive estimation