Py feature recipe wo probes (#768)

Fixes #760

Wraps arbor recipe (without probes, i.e. num_probes, probe_info, get_probe) including 

- cell_connection, 
- gap_junction_connection, 
- recipe with
  - num_cells
  - cell_description
  - cell_kind
  - num_sources
  - num_targets
  - num_gap_junctions_sites
  - event_generators
  - connections_on
  - gap_junctions_on
  - global_properties
- enum cell_kind in `identifiers.cpp`