Add frame of reference to MS_plot

[andreww]

It turns out that we don't document the plot orientation in MS_plot. We should and I recently sent this to a user:

In a sense the orientations in MSAT are up to the user. But if you generate a phase velocity
plot the directions are fixed. The positive X1 direction is up (12 o'clock), the positive X2
direction is to the left (9 o'clock) and the positive X3 direction is towards you, out of the
middle of the plot.

add this to the documentation