Zero-length cavity

[iagapov]

presently zero-length cavities do bot work due to code like this in in cavity_R_z (cpbd/r_matrix.py)
r_z_e = lambda z, energy: cavity_R_z(z, V=element.v * z / element.l, E=energy, freq=element.freq,...

zero-length cavity is usual feature in storage ring simulations, should be taken into account