Fix jump height

Changes to the formulas to calculate jump height

Fix for #229 (closed)

So far:

• Exponential interpolation used for linear damping in phys.rs::integrate_forces
• An approximation of the integral of the linear damping formula is used to adjust the position in phys.rs::Sys::run

It's a good approximation and allows for jumping two blocks at 15 fps, but could use some adjustment to make it exact. Possible solutions:

• fix the time step and run each tick multiple times per frame.
• find an exact solution to the (linear damping equation)[https://en.wikipedia.org/wiki/Damping_ratio#Derivation]
• Modify the existing collision code.