Allow MS_TI to handle eta

In MS_TI we Berkeley VTI paramaterisation ('panning'), but not the
more general VTI one.  Panning and Romanowicz in their tomography
models assume eta≈1, so you don't get the constants you expect if
you allow it to vary.

Add a routine for general VTI with all five parameters into MS_TI
activated with 'global'.
Add options to finess plots to MS_plot

We can now change the contours and FSW lines
to make nicer plots.