Reorganized bhtree methods, major code refactoring for readability and modularity.
Code refactoring for readability, some errors fixed in simulation creation.
Several constants added.
Clean pointer macro added.

The new clean_pointer() macro handles free’ing memory and ensuring no
dangling pointers exist.
Added proper skeletons for particle-particle and particle-node force calculations.
Minor changes, some reorganization.
Minor changes, some reorganization.
Modified heuristic skeleton.
Fixes one error with particle destruction, some changes for readability.
Major changes for readability, plus error with system destruction fixed.
Fixed error in partition find method.
Fixed error in vector destruction, some changes for readability.
Changed test simulation file.
Fixes some incompatibilities in testing with changes to simulation creation/destruction.