Slam: Add polymorphic policies, other changes

[publixsubfan]

Summary

Adds a policy tag type for selecting virtual/concrete interfaces when instantiating Slam objects.

• Two tag types are provided -- slam::policies::VirtualInterface and slam::policies::ConcreteInterface
• Supported Slam objects inherit from a "selector" type instantiated with the provided tag, which aliases to the correct base class (this is to support selecting between a CRTP class and an abstract base class):
  • OrderedSet/RangeSet: slam::Set if virtual, ConcreteInterface if concrete
  • Map/SubMap/BivariateMap: slam::MapBase if virtual, ConcreteInterface if concrete
  • ProductSet/RangeSet: slam::BivariateSet if virtual, CRTP ConcreteBivariateSet if concrete
• Adds conversions for Sets and BivariateSets between corresponding concrete and virtual interface types

Also:

• Access the underlying sets of a BivariateSet directly from the derived classes. For ProductSet, these are stored as member variables, and for RelationSet, these are accessed from the underlying relation.
• PositionSet and RangeSet now are just template aliases of GenericRangeSet
• Uses underlying Relation set types as default set template arguments in RelationSet
• Adds set template arguments to DynamicVariableRelation to correspond to StaticRelation
• Adds an allocator ID parameter to BivariateMap constructor

[agcapps]

General comment: this good. It's also in very strong need of documentation showing why you would choose one or the other, and how to use one or the other.