Teach the split_model about new eff split mode

Add optional arguments:

* split_model("mode", eff_split_mode) where eff_split_mode is a
  string passed into the mode argument of MS_effective_splitting_N.
  Note that setting this to 'GaussianWavelet' results in a much
  longer execution time for the example and the remaining options
  can be used to limit this.
* split_model("min_azi", min_azi) the minimum azimuth considered for
  the calculation (default 0 degrees).
* split_model("max_azi", max_azi) the maximum azimuth considered for
  the calculation (default 360 degrees).
* split_model("del_azi", del_azi) the azimuthal spacing used for the
  calculation (default 1 degree).
Seperae and tidy up trace plotting functions

This merges the final common functions from
MS_effective_splitting_N and MS_splitting_misfit
and provides a framework for plotting 'traces'
used for some splitting calculations.