Save current know working output from gravmag.transform for testing

[leouieda]

One way to test against a regression in the code is to save the current output that we know works and test our functions against it. This will be useful for functions that are hard to test against analytical results, like some in gravmag.transform and the forward modeling functions.

The saved outputs should be small and be kept in a fatiando/tests/data folder. These files should be installed with the package (will need to do something in setup.py for this).