Use member functions with NbnxnPairlistCpu and NbnxnPairlistGpu

No functionality changes, just moving free functions to be class members. This will make it easier to template NbnxnPairlistGpu and if necessary declare extern template definitions for all its methods.

Member function names now conform to style. Some variable name changes to avoid shadowing class definitions, since we have not yet got enough encapsulation to warrant making member variables private and suffixing them with underscores.

Changed "simple" to "plain C" when that was the intention, to avoid ambiguity with simple vs GPU pairlists.

Refs #3847