Update computation of accuracy so it's consistent with best practices

[paulalbert1]

Right now, accuracy is calculated as the average of precision and recall.

We should change it to: (true positive + true negative) / (true positive + true negative + false positive + false negative)

[paulalbert1]

Here's the proposed code for the change. This didn't work, but hopefully it gives you the idea. https://github.com/wcmc-its/ReCiter/commit/19dadab826a335a8cc6eda62e9f5e264df662020#diff-787324bc47fdd0a159b58f738bf98665

[paulalbert1]

This PR should cover it. We need to test to make sure this works as intended. https://github.com/wcmc-its/ReCiter/pull/512