Comments seismic

OMG... this is my first pull request.
And I gave up on the equation for attenuation correction in the docs, I couldn't get that to work.
Also, I made some unimplemented functions private, so they wouldn't show up in the doc, but there might be a better way of doing that.
I am pretty sure this only works for a single depth (the same is true for all other functions except evaluate_all_at() and internal_depth_list()