Add TransmissionLine block

[milanofthe]

Summary

• Adds TransmissionLine block: delay-based scattering two-port model
• Uses a single vector-valued AdaptiveBuffer from pathsim core to delay both wave directions simultaneously
• Crossed output: b1 = T * a2(t-tau), b2 = T * a1(t-tau)
• Parameters: length, er (permittivity), attenuation (dB/m), Z0
• Makes RFNetwork import graceful (try/except) so skrf incompatibility doesn't block the rest of the package

Design

• Inherits from Block, implements sample() and update() (same pattern as core Delay block)
• __len__ returns 0 (no algebraic passthrough — output depends on past only)
• Derived quantities computed on init: vp = c0/sqrt(er), tau = L/vp, T = 10^(-aL/20)