First implementation of a SingleParticle interaction. Includes a
SingleParticlePotential (and associated template), an interaction template and
as a first use case, a HarmonicTrap potential.

The python wrapper classes are not included yet, but the code (including the
registerPyton methods) compiles properly.
Add missing inline implementation of getMaxCutoff.
Add Python wrappers for SingleParticlePotential, HarmonicTrap and
SingleParticleHarmonicTrap.
Add a Lennard-Jones 9-3 SingleParticle wall potential.
Fix typo (missing period in .def).
Transform std::cout warning to use LOG4ESPP_INFO
Set LennardJones93Wall direction to 0 (x-dir) for consistency with XDensity observable.
Addition of SlabBC, a slab boundary condition in the direction 0. With respect
to the Orthorhombic BC, it does not perform the "minimum image" in the x
direction.
Change passing of position in SingleParticlePotential to passing of particle.
Allows to access all the particle properties in the potential.
Transform the SingleParticlePotential LennardJones93Wall to work with multiple species.
Document get/set Params, add rst file for user guide and example use.
edit documentation.
Enforce same ordering for interactions.
Add r0 parameter for type-dependent wall origin.
Merged in pdebuyl/espressopp_upstream (pull request #2)

Addition of SlabBC SingleParticlePotential for external forces.

Acknowledge by Torsten