Add support for multi-element channel modeling.

[capn-freako]

Currently, we allow for only a single Touchstone file for channel modeling.
This puts the onus of final channel model assemblage on the user, instead of on the tool where it belongs.
Ideally, the user would be allowed to specify separate Touchstone files to represent:

• Tx package,
• Tx card,
• Tx connector,
• back-plane,
• Rx connector,
• Rx card, and
• Rx package.

All elements should be optional.
And PyBERT should perform the final assemblage, using the routines available in the SciKit-RF library.

As per #145 , PyBERT should allow multiple files to be selected for any channel component above and automatically sweep through all possible permutations when the user has selected more than one file for one or more of them.
(Optionally, allow the user to select from the full permutation set, allowing simulation time to be reduced while still maintaining a record of all channel component variants tried.)