Let $string_map record both position and duration

Index | Description
  1   | String ID (1~6 for a typical guitar)
  2   | Attack time
  3   | Note data (position/duration)
Let latencies be measured in milliseconds
Count unit of duration in milliseconds

[!] The value must be casted back into integer, or it becomes float.
Add class guitar_string

This class defines the action taken with previous and current state of a
string.
Construct time-to-comcknd mapping

Output is a 2D array, where the first index is the operating time. The
policy of optimization is finding the latest operating time so far.
Print formatted output for USART transmission
Let the first key of $command_map be the origin

The original $command_map may have negative keys, because its 0 is the
time that the first note is played.