unused references

[lheagy]

In the upgrade to the most recent sphinx in #546, I found that there are a whole suite of references that are not currently being cited in em geosci. This raises warnings in the build process. I have moved the references to unused_references.bib. Presumably, they were added because we intended to cite them. So, we need to go back and figure out where each of these should be referenced and add them back to the main references.bib file.

First, we should search for the citation key throughout the site to see if it is simply a syntax error and the link is not being generated. For those remaining, we will then have to figure out where we intended to cite them.