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.
We read in the file every time you ask for a gravity? (also note that this won't work with an array of depths)