== MS_effective_splitting_N

 MS_EFFECTIVE_SPLITTING_N - N-layer effective splitting operator calculation.
 
 // Part of MSAT - The Matlab Seismic Anisotropy Toolkit //

 [fast_eff,tlag_eff]=MS_effective_splitting_N(f,spol,fast,tlag)
  
  Inputs:
     f (scalar) : Dominant frequency of wave 
     spol (scalar) : initial source polarisation
     fast (scalar/vector) : fast direction(s) of layer(s) to be included
     tlag (scalar/vector) : lag time(s) of layer(s) to be included

  Fast and tlag can be scalars or vectors but must all be the same length.  

  CAUTION! The method gives unstable results when
  the source polarisation is near the effective fast direction. This would,
  however probably be seen as a null result anyway.

  Reference: 
      Silver, P. G. and Savage, M. K. 1994 "The interpretation of shear-wave
      splitting parameters in the presence of two anisotropic layers" 
      Geophysical Journal International, v119 pp 949-963.  
