PWM class "stop" method #7

Hi Leapo,

First off, thanks for taking the time to write this python library!

I was playing around with it on my Rock64 SBC and noticed the following:

1. The GPIO.PWM.stop() method does not work as intended.

The PWM.stop method uses the multiprocessing.Process.terminate.
This approach stops the process, but does not respect the pwm_process static method try/except clauses, as such when calling the stop method the var_gpio_filepath file is written with the last value that was set by the pwm_process before calling it.
As a result, when calling PWM.stop the output GPIO PIN is set erroneously to 1 part of the time.

I have addressed this issue here.

2. pwm_busywait method proves too CPU intensive, constantly taking up 100% CPU time.

I have set the default 'pwm_precission' arg to 'LOW', as the linux kernel is able to achieve a acceptable error margin on its own using time.sleep,