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.
can you move burnman.tools.read_table("input_seismic/grav_for_PREM.txt") into __init__ so that it won't be called every time the user calls gravity?