Addressing issue 52

The *Accessor classes were originally members of AuxElement, the idea being that derived classes might want to change the way they work. However, there does not seem to be in fact any use case for derived classes changing these implementations. Further, having them as members of AuxElement prevents specializing them without changing AuxElement — and this is something we do want to do.

So they were eventually moved to be independent classes, with typedefs injected into AuxElement to avoid breaking the large amount of existing code that references them via AuxElement. As of yet, there hasn’t been to migrate this old code, but it would be better for new code to reference SG::*Accessor.

The guidelines should reflect the best practice rather than the legacy syntax.

Closes #52 (closed).