Improve gravity system

[KThompson-Lane]

Gravity is really finnicky and uses a lot of magic numbers. This should change.

[KThompson-Lane]

In #24 there were some changes to the gravity system to make it more abstracted and dynamic. However this is really quite a crude over-simplification and would be much better to actually model real physics. Though this is time-consuming and there are more pressing issues at present. As such moving priority to low and putting type to stretch.