v2.9.1

Bug fixes

• Fixed a minor bug where the variable period timestep would be off by
  one when the timestep got sufficiently large.
• Updated collections API to hide DeprecationWarning.
• Fix scaling of cutoff in Gay-Berne potential to scale the current
  maximum distance based on the orientations of the particles,
  ensuring ellipsoidal energy isocontours.
• Misc documentation fixes.