Fix problem where the screen attribute was not set on Atom instances if the
SCREEN section was not present (e.g., old prmtops)

Add NNB to the pointers dict as an alias for NEXT
Number of small fixes:

    - Allow the "chamber" command to set an implicit solvent radius set (do not
      require users to use the 'changeRadii' command afterwards)
    - Delete the combineMolecules action -- it makes far less sense with
      "autoimage" available and I've changed my mind about ever recommending it
      to others
    - Minor fix in the API so True and False can be passed as arguments without
      having to cast to a string first.
    - setOverwrite does not need a parm loaded to work
    - Flush changes from changeRadii to the topology data structures immediately
    - Fix default value for solute_ions argument in setMolecules action
    - Move netCharge activity to the __str__ method (and return the net charge
      upon execute()) to make it consistent with other introspective actions
    - Greatly simplify the addExclusions command (and should also make it
      faster)
    - Remove printouts from the deleteDihedral action's execute statement (much
      more API-friendly)
    - Fix the deleteBond action.