New isotopes

[lparsons]

• Adds support for additional elements (closes #5)
• Updated test data to account for the new approach of handling non-tracer elements (closes #11)
• Added CHNOSZ dependency
• Got rid of warnings issued due to use of tidyverse style data variables (see https://dplyr.tidyverse.org/articles/programming.html)

NOTE: I rebased this branch off of PR #10 so that it builds upon those changes and doesn't add conflicts. Please merge #9 and #10 BEFORE reviewing this. Also, this takes the place of PR #12 (it includes all of those changes, rebased).

[lparsons]

@XiaoyangSu Please be sure to look at the updated test data. The new files in this PR have been run with the updated code. You can compare them to the old files (they are all Excel files, so you'll have to compare them outside of GitHub using Excel). If could make sure they are accurate, that would be very helpful.