Add problem 18 description (units)
Add spacial units definitions: Game, Pixel, Tile
Add time units, switch to its using: Acceleration

Instead of doubles, velocity and acceleration now measured in Velocity
and Acceleration units correspondingly.
Remove Milliseconds unit, it is not really needed

std::chrono::milliseconds suits our needs perfectly
Use units::FPS instead of ints
Huge refactoring: use units instead of int/double