Use absolute values to calculate normalizing factor for relative norms in adaptivity

[IshaanDesai]

As the title states, absolute values of quantities need to be compared while picking the maximum value from a pair of values, to use as the normalizing factor for relative norms in the adaptivity computation. This is important when two negative values, or a negative value and a zero value are being compared in the adaptivity computation. @mathiskelm pointed this out.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[mathiskelm]

Looks good, thank you.