Initial geometry added

seems to display OK with basic geometry (no stripe)

no behaviour yet
Charge class made

specific class made for charge object (mainly for interesting animations
later)
charge and cart displaying correctly with track

no behaviour yet.
mass adjustment working

mass adjustment working via vertical dragging of upper edge of cart.
Basic charge behaviour works

Very basic graphics but it works.
Merge pull request #1 from Cheddles/initialCharge

Basic charge behaviour works
deleted stripe

got rid of stripe as couldn't make it look good. Using cart width
instead (more intuitive and cleaner-looking)

Started implementing levels
Win condition checked OK

scoring also works.
may need to trigger loss condition after only one cart has exited the
screen.
Merge pull request #2 from Cheddles/checkWin

Check win