This PR implements basic transient models for simple transmision lines. The list of devices with added model:

• Microstrip line
• Coaxial line

Here is a test schematic in Qucs-S GUI. The TRAN model for coupled lines is not implemented. The help is appreciated here.

Limitation of the models:

• The ripple on the pulse edges are not calculated. The existing models for ideal TLine also don't calculate ripple.
• It's unclear how to calculate losses. The losses for MS lines are derived from a fixed frequency which is unknown for transient.