Fix typo that set force where potential exceeds max_potential to max_potential rather than zero; add partial support for potentials acting on angle between two vectors

This reverts commit 00449a36.

Use scipy.interpolate to remove NaNs from interactions; thank you to Pin-Yi Li for suggesting interpolation :)

Improve IBI by adding progress bars; compute target distributions from optional target_universe in run_IBI; allow replicas; separate max_force and out_of_bounds_force; and scale IBI updates by (rho_target/rho_target_max)**(0.25) to reduce instability caused by noise where the signal is low

get_bonds() and similar copy bonds/angles/dihedrals/exclusions lists; potentials return self for __copy__ and __deepcopy__