Clean-up and update fatiando.seismic

[leouieda]

In establishing a formal API for fatiando.seismic following #353, I'm hoping to remove any unused/useless code that has been left there. This is the list of things that I think don't need to be in Fatiando. Remember that less code means less bugs and less maintenance.

• epic2d: this was programmed for a short course on inverse problems back in 2011. It's not useful in practice and hasn't been touched in years. It's relatively simple and could be easily inserted into a notebook or something if someone needs this for a class.
• profile: this is the same as epic2d. I think people actually do straight-ray VSP inversions but I'm not sure. If it is useful, we would need to update this to the point where it would be easier to re-implement the whole thing with tests.

I'm a bit conflicted with srtomo because I've seen papers using straight-ray tomography for cross-hole problems. I'm inclined to keep it but merge in the forward problem from ttime2d into a single module.

The finite difference code is useful and being updated at a glacial pace #308.

The convolution code needs some cleaning but it's very useful as well.

@fatiando/contributors any thoughts on this would be greatly appreciated. If no one objects, I'll delete epic2d and profile and clean up the rest of the code.