Make AI update take time into account

[Clockwork-Muse]

Issue description

Currently, the contrib ai system doesn't take the time between updates into account, which means that ai-powered objects move slower at lower frame rates (and faster at higher ones).

The way facing updated would, if the object was supposed to move more than ~5 meters per update, make the object face backwards to the direction of travel.

Solution description

• Corrects facing to be based off the current position and force direction, instead of the previous position.
• Applies time to velocity/position calculations.
  • If a current project is using a fixed 30/60 fps no updates are likely to be required, but outside of that delta-time should be threaded through, and object mass/force updated as appropriate. Frame rates below ~10 fps may interact poorly with the physics system due to no substeps.

Checklist

I have done my best to ensure that…

• [x] …I have familiarized myself with the CONTRIBUTING.md file
• [x] …this change follows the coding style and design patterns of the codebase
• [x] …I own the intellectual property rights to this code
• [x] …the intent of this change is clearly explained
• [x] …existing uses of the Panda3D API are not broken
• [x] …the changed code is adequately covered by the test suite, where possible.