Add support for *.s8p and *.s12p channel model data.

[capn-freako]

Currently, we only allow *.s4p Touchstone files to represent the channel.
This precludes incorporating cross-talk in our channel analysis.

==> Expand PyBERT's channel modeling/analysis to include support for *.s8p and *.s12p Touchstone files, to allow for including cross-talk in the channel analysis.

[capn-freako]

Note: Slight change of plans here: let's keep the "N" in *.sNp variable and the mapping of the different differential through paths of the incoming Touchstone file flexible.
(Probably, just allowing the user to identify the "victim" is enough.)