Fix typo.
Only check Indexes in debug mode.

Previously these checks were really slowing down evaluation of
close pair scores in 'release' builds (for a typical application of
ligand_score Index::get_index() accounted for 23.2% of all CPU time).
Link to correct class.
Fix typo.
Provide at() as well as operator[] for vector-like objects.

Similar to std::vector, at() does bounds checking and operator[]
does not. This allows us to speed up operations on ParticleIndexes
in release mode by skipping a lot of unnecessary bounds checks.