Percentage Price Oscillator (PPO)

``` use ppors::PPO; use tacommon::traits::Indicator;

let mut ppo = PPO::new(2, 5); asserteq!(ppo.next(81.59), None); asserteq!(ppo.next(81.06), Some(-0.21699967245331922)); asserteq!(ppo.next(82.87), Some(0.5209675887612042)); asserteq!(ppo.next(83.00), Some(0.6190403299147216)); asserteq!(ppo.next(83.61), Some(0.7468846224456406)); asserteq!(ppo.next(83.15), Some(0.4239424161439804)); asserteq!(ppo.next(82.84), Some(0.13356017757692787)); asserteq!(ppo.next(83.99), Some(0.4997247953737079)); asserteq!(ppo.next(84.55), Some(0.690806261787222)); asserteq!(ppo.next(84.36), Some(0.503265546395477)); asserteq!(ppo.next(85.53), Some(0.8097662523725208)); asserteq!(ppo.next(86.54), Some(1.0883304662225042)); asserteq!(ppo.next(86.89), Some(1.039770979517235)); asserteq!(ppo.next(87.77), Some(1.1327352631554244)); assert_eq!(ppo.next(87.29), Some(0.7158876517890078)); ```