Self-avoiding walk/overlap check

[chrisiacovella]

As a new feature, we might want to write a simple function that checks for overlaps between two compounds. This would require knowledge of the characteristic diameter (and an option to exclude bonded objects); I suppose diameter could be passed as some dictionary (or assumed to be 1 if not given). My initial thought for this is for generating a long, non-linear polymer (i.e., have it take a self-avoiding walk), but this may also be useful for avoiding really bad starting configurations for most systems.