Migrate from StateBlock to StateDerived

Following the bootstrapping addition, #461 (closed), StateBlocks must be constructed as derived objects, and StateBlock should become an abstract class.

• Make StateBlock::transform() = 0 pure virtual
• Substitute all instances of StateBlock by appropriate derived types:
  • StateParams<size>
  • StatePoint2
  • StatePoint3
  • StateVector2
  • StateVector3
• Fix FactoryStateBlock to produce always derived blocks, of the correct type according to
  • key e.g. 'P' is StatePoint, and we do not know the size yet
  • type e.g. "StateBlockVector3" is StateBlockVector3d
  • size e.g. 'P' with v=Vector2d(...) is StatePoint2d