MS_SPLITTING_MISFIT - Calculate the misfit between a pair of splitting
                      operators.

Usage:
    [misfit] = MS_splitting_misfit(fast1,tlag1,fast2,tlag2,spol,dfreq)
       Calculate the misfit between splitting operators [fast1,tlag1]
       and [fast2,tlag2], using the default mode (see below). SPOL is
       the initial source polarisation, dfreq is the dominant frequency.

Modes of operation:
    [misfit] = MS_splitting_misfit(...,'mode','lam2')
       Calculate misfit based on the residual second eigenvalue of applying
       [f1,t1] to a test wavelet, then removing [f2,t2]. [DEFAULT].

[misfit] = MS_splitting_misfit(...,'mode','lam2S')
   Calculate misfit based on the residual second eigenvalue of applying
   [f1,t1] to a test wavelet, then removing [f2,t2]. This is then
   reversed, and the final misfit is the average of the two.

[misfit] = MS_splitting_misfit(...,'mode','intensity')
   Calculates the misfit as the difference between the splitting intensity
   (see, e.g., Long and Silver, 2009) for the two operators.

[misfit] = MS_splitting_misfit(...,'mode','simple')
   A simple (normalised) arithmetic misfit.

References:
    Long, M. D., & Silver, P. G. (2009). "Shear Wave Splitting and Mantle
      Anisotropy: Measurements, Interpretations, and New Directions". Surveys
      in Geophysics, 30(4-5), 407–461. doi:10.1007/s10712-009-9075-1

See also: MS_EFFECTIVE_SPLITTING_N