reduce taking time for streamtube function

[gilbertogalvis]

This PR reduce time elapsed when fig2plotly works with streamtube matlab function

Set TreatAs Optional parameter to streamtube is needed.

Example code to test:

load wind
[sx,sy,sz] = meshgrid(80,20:10:50,0:5:15);
streamtube(x,y,z,u,v,w,sx,sy,sz);
view(3);
axis tight
shading interp;
camlight; 
lighting gouraud

fig2plotly(gcf(), 'TreatAs', 'streamtube');