Clean up bibtex file

[bambooforest]

When merging all the new inventory resources, the process introduced duplicate bibtex keys (some from the original data providers themselves).

TODO:

• remove duplicates
• double-check that all inventory ID's have a bibtex (currently at least two from ER are without citations; others are URLs to online materials which need a reference added)
• make all entries valid bibtex format (when using the R bibtex package, it spits out a bunch of warnings where certain required fields are missing)
• add the relevant tests for this stuff to the tests directory

[bambooforest]

PR #168 handles removing duplicates.

[drammock]

cc #210, which did some of this. Still need to validate / implement tests.

[bambooforest]

cc #240 which found some errors here.

also, a thorough clean-up would also deal with possible mismatches (as found above) between:

• filenames for squibs in GM inventories
• and filenames for squibs in GM bibtex entries