\brief Composable functors to implement scores via compile-time composition.

Currently, there is support for distance based IMP::PairScore types generated from the IMP::score_functor::DistanceScore and IMP::score_functor::DistancePairScore. Angles and others can be added if desired.

No functionality is exported to Python. It can be done with some work, but it
is not clear that it would be very useful.

# 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).
