Remove duplication in MS_plot

This is in preperation to dealing with
issue #6.
Add plotmap argument to MS_pole

%     MS_plot(..., 'plotmap', pmap)
%          Change the order, or content, of the subplots. This allows a
%          single pole figure (e.g. the P-wave velocity) to be drawn, or
%          the pole figures to be drawn in a different order. The argument
%          pmap must be a cell array of strings. Each string represents a
%          different pole figure and its location in the cell array
%          represents the location of the pole figure. Thus the default
%          {'VP', 'AVS', 'AVSPOL'} draws three pole figures in a row. If
%          the argument were {'VP'; 'AVS'} two pole figures would be drawn,
%          one above the other. Valid strings are 'VP", 'AVS' and 'AVSPOL'.
%          These are not case sensitive.
Allow V_S1 and V_S2 pole figures to be drawn

Plugs into plotmap.