Add features for working with S-parameters

[sgherbst]

Summary

This PR adds some functions for working with S-parameters, such as calculating step and pulse responses for measurements in a *.s4p file. The code is adapted from earlier work in the hslink-emu repository.

Details

• Added a new channel_type for the Channel class called s4p. This can be used to construct a channel from S-parameter measurements. Example:

  chan = Channel(channel_type='s4p', sampl_rate=10e12, resp_depth=200000,
  s4p='myfile.s4p', zs=50, zl=50)
  _, pulse = chan.get_pulse_resp(f_sig=16e9, resp_depth=350, t_delay=0)

• At a lower level, added the function s4p_to_step(s4p, dt, T, zs, zl)
  • s4p is the name of the *.s4p file
  • dt is the desired time increment between points in the step response
  • T is the duration of the step response
  • zs is the source impedance (optional argument, defaults to 50)
  • zl is the load impedance (optional argument, defaults to 50)
• Added function s4p_to_impulse with the same arguments. For the pulse response, use the get_pulse_resp function of the Channel class.
• Fixed non-ascii characters in Case4_FM_13SI_20_T_D13_L6.s4p