Resolve issue with heatmap color scaling with negative results

[marc-vdm]

• Resolve #1210
• Was due to scaling being based on max, while it should have been based on the max of the absolute values.

Previous:

Current:

And when positive and negative results are present:

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [ ] Update the documentation, please follow the numpy style guide.
• [ ] Update tests.
• [x] Categorize the PR by setting a good title and adding one of the labels: bug, feature, ui, change, documentation, breaking, ci as they show up in the changelog.
• [x] Link this PR to related issues by using closing keywords.
• [x] Request a review from another developer.

[coveralls]

coverage: 50.933% (-0.01%) from 50.944% when pulling c4fd7807863b02378ea94bd99f0cbbea09c28644 on marc-vdm:result_heatmap_scaling into 17f432a87f313a5f29c42bebc3e4e9c41be63e7c on LCA-ActivityBrowser:master.

[mrvisscher]

Looks good!