Gravity for player and pixels

[korysanchez]

Create a gravity force, which is a trait that can be inherited by the player, and any pixel elements. should mostly match real gravity, i.e. will accelerate towards 9.8 m/s.

[Chisato243]

1m = ??? pixels

[korysanchez]

lets say 1 for now

[korysanchez]

The calculations for gravity are done in the player file, but we should abstract the calculations to be done in the gravity.rs file. We'll leave this to a future date however, as it's currently functional for the player.