Fix spelling, grammar, typos.
Fix test so that it doesn't fail randomly.

Previously, a rigid body would ensure that an interparticle
distance is invariant; however, the reverse (that the lack
of a rigid body means the interparticle distance will always
change) is not true. Try to explicitly separate such particle
pairs; this will fail if the rigid body is still active.
Speed up test; use correct residue indexes.
Speed up by removing unused RMF output.