Add "Used by" section

[santisoler]

Would be nice to have a section that lists all published works that make use of Fatiando libraries. This way users can see other possible usage examples of Fatiando besides gallery examples. Also this could open the door to show how developing Fatiando actually saves us time when researching and writing papers.

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[santisoler]

#15 is adding a section for listing all software that make use of Fatiando libraries. Would be nice to add another section (or maybe divide the "Who is using Fatiando?" one into two) that lists scientific articles that make use of Fatiando.