Add several ParetoLocality classes to pheet namespace
Refactor VirtualArrayBlock

Member names, array initializer, replaced getters/setters by public
field.
Simplify VirtualArray ctor
REBASEME: constexpr + const
Refactor VirtualArray::swap

There's really no reason to call find_block four times.
Alloc the VirtualArray directly within the PLTSPlace

This simplifies memory management and makes a couple of statements such
as usages of operator[] less ugly.
Initial VirtualArray iterator code
Use the iterator in top()