开关磁阻电机控制 库函数

代码说明：

 This simple library allows control of switch reluctance motors such  as those found in hard disks. It provides varying degrees of precision  assuming three phases.    Library also supports rudimentary speed control which allows programmers  to control motor speed without resorting to usage of `delay`. This is done   through setting speed measured in revolutions per minute and calling  `motor_step` or `motor_step_backward` frequently. State of pins will be  updated only after specific time has passed. Speed control does take into  account sequence sizes, so no additional calculation is needed.