This patch adds particle refines to IMP. There is a base class in <IMP/
ParticleRefiner.h> and a couple in <IMP/particle_refiners>. The
current particle refines cover an bond with a set of spheres (created
on the fly) and move down one level in the hierarchy. I added some new
meothods and classes to the python test base class too.
SWIG was having problems with figuring out that Particle Refiner was
not a virtual class when I had all the methods be virtual, so they are
now all non-virtual.