Introduce new "severity" column in comparison

[rylev]

Especially with the introduction of a mechanism for deciding on a non-hardcoded significance threshold in #996, it is nice to know by what factor a particular test result is over the significance threshold. I propose to add a "severity" column which shows by how much the test result is over the significance threshold.

For example, for a test result of 1.5% and a significance threshold of 0.5%, the "severity" would be 3.0.