Add serial port flow control function

[bonner72]

Add serial port flow control function.

[bonner72]

Not sure if this is a possible feature with current serial code.

[bonner72]

https://hades.mech.northwestern.edu/index.php/Processing#:~:text=The%20serial%20library%20contains%20the%20data%20type%20Serial%2C,serial%20comm%20port%2C%20write%20and%20read.%20import%20processing.serial.%2A%3B
This article provides a modified library with support for flow control. Have not tested it yet.