Support chaining of channel files

[savob]

Consider allowing the chaining of channel descriptions (.s4p/.mat) to allow link systems to be more flexible. For example allowing one to specify a die-to-package, package-to-pad, transmission, and then a pad-to-die channel. There effects/responses could be combined and used in the simulation.

This would allow one to change the transmission line but keeping the same die-to-pad channels for a system to see how the system hardware can be tweaked.