obj = CLinearEffects()
  obj = CLinearEffects(Name, Value, ...)
  Creates a process representing linear effects not connected
  to absorption or gain, i. e. dispersion and diffraction.
  It basically represents the:
    i (k_z(\omega, k_x, k_y) - k_0 - \omega / v) A(\omega, k_x, k_y)  
  term in UPPE
  The options are:
  'LinearPropagation': 'on'(default)/'off' - no linear effects
                       for 'off' 
  'SpatialEffects': 'on'(default)/'off' - diffraction and Walkoff
                    can be switched of with this option, while
                    dispersion stays on.
  'BackPropagation': 'on'/ 'off' (default) - perform propagation
                     in the negative z direction. Usefull for 
                     preparation of convergent beams.
  'WindowVelocity': value [m/s] / 'Average'(default) - sets
                    the velocity (v - from the equation above),
                    of the time window in which
                    the pulse is located. If set to 'Average'
                    the average group velocity of the fastest
                    and slowes envelope (at their ref freq) is
                    taken. Otherwise the numerical value of
                    the velocity can be set.
  'ReferenceWavenumber': 'on'(default)/'off' - this option
                         alows disabling the use of the reference
                         value of wavevector (k_0 in the
                         equation above). Note! This will not
                         remove the phase mismatch terms in the
                         nonlinear processes. Effectively if
                         this option is used, the phase
                         mismatch for most nonlinear processes 
                         will be doubled. Use this option with
                         specialy designed processes only.