\brief Various classes to hold sets of particles.

Containers provide a layer of indirection between code that defines sets of particles and restraints and constraints. This indirection allows the set of particles acted upon to be changed on the fly. For example, the IMP::container::ClosePairContainer defines a set of all particle pairs that are close to one another.

# Info

_Author(s)_: Daniel Russel

_Maintainer_: `benmwebb`

_License_: [LGPL](https://www.gnu.org/licenses/old-licenses/lgpl-2.1.html)
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2 of the License, or (at your option) any later version.

_Publications_:
 - See [main IMP papers list](@ref publications).
