documenting and polishing classes 

There it seams there was some bug when the fatiando repository was moved from leouieda\fatiando to fatiando\fatiando. My repo couldn't get the new branches you created. So to make it work I had to use your  recent master version copy it and rename it to wavefd-simulation-class. After all that some new thing I faced while working over the simulation classes. 

I don't want to be hard with python but it seams its documentation features for abstract/interfaces/derived class or methods lack functionalities.  For documenting methods that are going to be used by derived classes I had to appeal to [this](http://stackoverflow.com/questions/2025562/inherit-docstrings-in-python-class-inheritance) `DocInherit` class made by someone else from stackexchange. 

I also changed `spacing` to `dx, dz` where I found it and tested it. 
I added the GaussSource and was trying to figure out how to write the test cases after watching the software carpentry lessons.

Well this is just a begging I will push more stuff as they come. Also wanted to share the stuff about documenting abstract classes.