周期中位数（MD）

 use ta_common::traits::Indicator;
 use md_rs::MD;

let mut md = MD::new(5);
assert_eq!(md.next(81.59), None);
assert_eq!(md.next(81.06), None);
assert_eq!(md.next(82.87), None);
assert_eq!(md.next(83.00), None);
assert_eq!(md.next(83.61), Some(0.8807999999999993));
assert_eq!(md.next(83.15), Some(0.6712000000000102));
assert_eq!(md.next(82.84), Some(0.2287999999999954));
assert_eq!(md.next(83.99), Some(0.3855999999999938));
assert_eq!(md.next(84.55), Some(0.5135999999999938));
assert_eq!(md.next(84.36), Some(0.6263999999999982));
assert_eq!(md.next(85.53), Some(0.6712000000000046));
assert_eq!(md.next(86.54), Some(0.8327999999999974));
assert_eq!(md.next(86.89), Some(0.9127999999999957));
assert_eq!(md.next(87.77), Some(1.0184000000000082));
assert_eq!(md.next(87.29), Some(0.6152000000000072));