Migrate cell_group and fvm_cell to internal API.

* Define an internal API for lowered cells.
* Devolve all responsibility for index
  translations to `cell_group`.
* Reduce state/info required in lowered cells,
  for example probe information and cell segment
  indices.
* Fix (to confirm) issue where a cell could have
  synapses of only the one type.
* Spike detectors now keep a lowered cell-specific
  handle instead of a segment location.