debugger now enlist destination sites correctly for diffusion reactions.
make it so that the concentrationwall works with a target number of
particles instead of a concentration.
fixed a bug with the concentration wall. Now it's starting to act like
it's supposed to.
basing object ID on the refcount turned out to be a bad idea. Lead to
undefined behaviour in the ordered set due to different objects pointing
to the same ID. Added a separate counter for the IDs, which solved the
bug as well.
All tests pass. Task complete!