Add spike counts to pre-commit tests. (#1965)

- Enhance `run_cpp_examples.sh` to do some very basic checking of spikes counts
- Add a script to check all releases
- The former must succeed for all commits to master

Here's the expected spike counts as of 0.7
- brunel 6998
- bench 972
- ring 94 (19 pre-v0.7)
- gap_junctions 30