correct stimulus for RC filter boundary tests
make rc filter processing width configurable

because rounding errors in the RC filter accumulated too big, there was
the decision to seperate interface width and processing width.

a rule of thumb is: be 20dB better than your incoming signal, therefore
win in dB = N*6dB + 3dB
and we should be at least 3 bit better in processing.

for double safety, we use 4 bits in the testbench and the data slicer
from now on.

testbench final value error went from >20LSB to 1LSB