Refactor CT->RT Vector, General Half-CT Mapper

- follow up to #335
- satisfy @psychocoderHPC at
  https://github.com/ComputationalRadiationPhysics/picongpu/pull/347/files#r11628952
Fix Interface To linearIndex
Merge pull request #349 from ax3l/topic-refactorRTMapping

Refactor CT->RT Vector, General Half-CT Mapper