Scalar add safety checks for #ON bits (w >= 21)

reenable checks found in SDRCategory. ScalarEncoder is widey used, ensure proper function with these checks as well. 
See ML discussion. 

w >= 3 is strict minimum for functionality. However w >= 21 is recommended for normal SP,TP functionality, and is required by this check. In special cases (such as demonstration demo, where you want users to read the bit-output) it's possible to avoid this check by passing `forced=True` to the encoder constructor.