Provide citation information for all functions/classes/modules

[leouieda]

We need to make a better job of providing citation information for Fatiando components. The coolest thing I've seen so far is duecredit. We can annotate our code with references and the user can use duecredit to generate a full list of references (including external libraries).

[leouieda]

#222 could use this maybe

[mtb-za]

So #293 seems to be working well. We should consider rolling this out for all the methods we have implemented.

As a general preference, do we want to use a DOI or a BibTeX fragment? Some methods may not have a DOI if they are in a thesis or similar, but to use BibTeX generally is more effort, and takes up more space.