Double check that all scripts we include in PyNeuroML have usage documentation included in them

- pynml
- pynml-channelanalysis
- pynml-modchananalysis
- pynml-plotspikes
- pynml-povray
- pynml-sonata
- pynml-summary
- pynml-tune

`-h` should work for all of them, and include information on their usage.