Improve performance of H5.unpack_data.
Improve performance of H5.unpack_data.

* Pre-load point ID -> group ID map to avoid repeated binary
  searches.
* Remove memoization (ID map takes care of caching now.)

BlueBrain/NeuroM,wizmer/NeuroM,eleftherioszisis/NeuroM,liesbethvanherpe/NeuroM,lidakanari/NeuroM,juanchopanza/NeuroM,mgeplf/NeuroM