Add an RF cavity model for traveling-wave linacs

[cemitch99]

The existing RFCavity element is designed to represent standing-wave RF structures.

The modeling of traveling-wave linacs requires a modified algorithm for tracking in the RF fields.
For example, this can be done by tracking in a superposition of two superposed standing waves with 90 deg phase shift, or by enabling Ez(z) to take complex values.

Both IMPACT-Z and IMPACT-T contain models of traveling-wave RF structures.
(In IMPACT-Z, this is type code 106. More details are provided in the IMPACT-T documentation.)

Additional options/examples:
https://doi.org/10.1109/TNS.1979.4330585