Add CSV example file for mutually_exclusive() opposites

[johnbradley]

Improve the mutually_exclusive function example to demonstrate how a user can use a CSV file of opposite IRIs.
See #270 (review)

[johnbradley]

Commit 2c1d347 roughs out the changes. I think the example CSV file needs to be fleshed out more.

[hlapp]

@wdahdul would you mind helping out with this by providing a few pairs quality terms deemed opposites of each other?

Ideally, these pairs would be contextualized to determining mutually exclusive phenotypes with and without providing opposites. In the newly added example, we are starting with femur phenotypes, but this could be changed. The first example in the manual page starts with the Dillman study and then randomly samples 10 phenotypes.

This would probably benefit from making it more deterministic, but such that we end up with a full range of exclusivity types, and that some of them would change after providing the list of opposites.