Accessors use more inheritance

[mtao]

Isolated the implementations of different sorts of accessor functions to increase clarity and utilize inheritance so non-const accessors can be implicitly converted to const accessors.

• AccessorBase does raw writing into attributes
• CachingAccessor utilizes attributes' caching abilities when available
• TupleAccessor disables index-based access and implements tuple-based access
• ConstAccessor enables const tuple-based access
• MutableAccessor enables const and non-const tuple-based access

Accessor<T,IsConst> now is a typedef for ConstAccessor or MutableAccessor depending on IsConst

Also, added protectedmember functions to let accessor friends cast to lower level accessors when necessary. These should be easy to identify / avoided in general code