Update selections every frame

[maxscheurer]

At the moment, the atom selections from MDAnalysis are only set once. However, for special selections as "around", this might change during the trajectory.
One needs to update the atomselections every frame to get the right contacts or surface areas. (btw @Astroprog , I'm testing the Issue tab on github!)

[ghost]

closed issue, selections are being updated

[maxscheurer]

Only update selection every frame, if "around" selection command is in selection text, so that the selection is much faster if it does not need to be updated.
Sorry for reopening though...