revise config timebase parameter / docstring
Initial doc for timebase (Time step between measurements is 10 ns * 2^(time_base)
) was based on 10MHz ps2000. New tests on a 25MHz ps2000 show the definition is different here.
Picotech docs say to consult the header file for timestep info, so perhaps we just grab the information there and revise out definition. If possible, make the system easier than just a blind integer.