chap_1: add fly exercise
chap_2: add helium exercise
chap_2: add objects with mass moved by wind exercise
chap_2: add gravity to mass_1 file
chap_2: add gravity and perlin wind exercise
chap_2: add an example with gravity, wind, and some friction
chap_2: add gravity example with 1 attractor
chap_2: add attraction as a mover method
chap_2: gravity - have all movers attract each other
chap_2: add bigger example of gravity
chap_3: add angular velocity example
lib: add polar to cartesian func
chap_3: use polarToCartesian func in angular example
chap_3: add spiral exercise