Add continuous linear focusing element

[austin-hoover]

This PR adds an axisymmetric continuous linear focusing node orbit.teapot.ContinuousFocusingTEAPOT.

• The tracking is identical to the linear component of the quadrupole (orbit.core.teapot_base.quad1), but it is focusing in both planes.
• It ignores the nonlinear component of the quadrupole (orbit.core.teapot_base.quad2), which depends on the energy offset from the synchronous particle. So this node should be used for coasting beams with zero energy spread.
• The focusing is axisymmetric; the phase advance is the same in both planes.
• There is no focusing in the longitudinal plane.
• Addresses Issue Continuous focusing element #62, but does not account for dispersion.

[azukov]

per @shishlo let's add if length is zero then do nothing check, otherwise it's good.