add troubleshooting FAQ

[davewhiteland]

Most obvious problem is JSON parsing errors, but it would be helpful to acknowledge the dependency chain too.

I've been caught out by ring_radius being smaller than radius so not showing up, for example.

[davewhiteland]

• don't iterate over the graph.travellers array if you're calling traveller.destroy() because it splices the array
• be aware that you can inadvertently specify identical keys in the JSON config, thereby silently overwriting previous ones (e.g., two vertices definitions)
• pulse will never work until has_pulse is true
• if you think journey_lifespan is set to 1 but it's really 0, you can have a huge memory leak (whoops)
• not specifying some of the named functions is a fatal error during setup/init, but others aren't (e.g., travellers' on_arrival missing is a run-time error) — need to look into this or have a policy on it
• creating a vertex with a non-unique id is not rejected as error but if you do that behaviour is undefined