Terraforming callback

[DaloLorn]

Per the 1.1.0 patch notes:

• The ProduceNativePressurePct hook will now correctly produce pressure for multiple resources (if applicable), and updates its pressure selection if the planet's resources are altered.
  • This update runs every 30 seconds. Future MP versions will instead add a new callback to notify hooks that the planet's resources have changed.

I'm turning this into an issue for the sake of visibility.