add digital filters

[jiangge12]

already add digital filters in hantekosc samples: https://jiangge12.github.io/Pic_DSO.png

it is quite easy by using scipy , only few lines:

from scipy import signal
b, a = signal.butter( 5, 0.075, 'lowpass' )
y_data = signal.filtfilt(b, a, y_data_in)

not familiar with openhantek6022, wish as simple as python