`atomselect` improvements

[stefdoerr]

1. Current implementation does bond detection using 3D histograms. For unwrapped trajectories those histograms end up huge and kill the process. Should rethink the implementation or at least throw a warning when performing atomselect on unwrapped trajectories
2. Speed it up by only doing bond detection when necessary. For example in resid 158 there is no need to calculate the bonds.