Add some unit tests

[leouieda]

Description of the desired feature:

This package was tested on its own code and other Fatiando projects but doesn't currently have any unit tests. It would be nice to have some for the parsing and editing code in burocrata/tests to get started. Testing the CLI is more complicated but if we can have some tests for the inner code it would already be great.

This will require moving the code from main() into separate functions that can be tested.

Are you willing to help implement and maintain this feature?

Maybe but it will take some time so if anyone else wants to try this I'd be more than happy.