Python: Doc strings and type checking
User interface for collision detection between three particles. Based on Gizem's work
test script
collision.cpp and collision.hpp are modified to define the different angle bonds with different angles.
Merge branch 'master', remote-tracking branch 'origin' into three-particle-binding

Conflicts:
collision.cpp, collision.hpp files that include different angle bonds with different angles.
Merge branch 'master', remote-tracking branch 'origin' into three-particle-binding